Gas Chromatography (GC) using columns of different polarities would be the recommended technique to determine the composition of a solvent system. It is quick, simple, and cheap. You could also use HPLC as one colleague has suggested but it is not as easy and precise as GC. Before using the technique for determining the solvent composition, you should use it first with pure compounds to see where component comes out (retention time) and then use it with the mixture. To get good separation and ultimately good results, optimize the temperature program first. For more information and to know more about gas chromatography, you could consult with text books in the field. 

The method to determine the composition can be a quick GC (Gas Chromatography) analysis which may take 20-30 minutes based on your instrument and temperature program which is crucial. NMR is also helpful if you going for estimations. Other methods to check chemically may be time-consuming and, for a say in your example, alcohol test for Methanol will be helpful. Can you run them and calculate the percent composition! 

If you do not need an exact composition and can use the solvent mixture within a range of composition in the recycling step (For fresh and quantitative estimations based analysis/separations/dissolution- use new rattioed solvent mixture), the other way is to roughly guess the contents percentage in the mixture as a good guess. This STRICTLY relies on the condition of your evaporation conditions of the temperature and pressure plus the vaporability of both the component solvents in the mixture. If you recover, in the given conditions of temperature and pressure (a read out on the rotavapor may be available which is in certain models from various manufacturers, although Buchi has performed well and is good one to use!) 100% of the solvents which is practically tough to achieve, the recovered solvents are at the original ratio. If one solvent is lost more, need to adjust that ratio based on combined quotient of the temp-pressure effect on the solvent although in mixture the effect will not be same as indicated for solo solvent effect on evaporation because of temp-pressure, it can be a workable guess. This info/ guess comes from the conditions of evaporation! Physical properties and behavior of solvents can be obtained from preliminary books on Chemical Technology dealing the subject. To the point, if you have water and hexanes (Hx) mixture, on rotavapor, you try to evaporate all Hx or the conditions are that way, all recovered solvent is water with some contents of methanol at 760 mmHg, 62 dC temp, solvent ratio 94:6:: Hx: MeOH, w/w).  

Check the solo solvents, binary and ternary azeotrope data, various concerned nomograph and alignment data for materials to know more:Check this out: The Chemist's Companion,Arnold J. Gordon, ISBN (hard copy): 0-471--31590-7

HPLC

Hi

The normal and simple method to determine the percent of solvent in a mixed solvent is to do simple distillation if the difference of the boiling points is big and to to fractional distillation (using fractionated condenser) if the difference of the boiling points is small. The solvent that have lower boiling point will distilled first, you can measure the volume of this solvent easily, then the second solvent will distilled at its  boiling point, you measure the volume of this solvent and so on if you have more than 2 solvents.

The most precsied method is using HPLC (more costed method).

May I know what is the solvent you are referring ? If you are searching for a quantitative test it would be the best using Gas Chromatography, if qualitative test you can look into the functional groups of your solvent mixture (if you are mixing organic solvent). 

For example : 

You can determine a phenol acetone mixture by chelating phenol with iron (III) and form a purplish compound, which later can determine using UV-Vis spectrophotometer. 

Methanol chloroform mixture can undergo acid base titration with aid of pH meter to determine methanol fraction. 

Conc. of solvent A (g/100g) =

[Measured conc. of solvent A (g/ml) x volume of sample (ml)]/ Mass of sample (100g)     

Actually a non-formal method was to do TLC with a reference sample. I practice this as a quick check of my solvent when I'm doing column chromatography. You can spot a reference sample on a TLC plate and develop it with a known ratio mixture of your solvent and develop another using your recovered solvent mixture. 

Based on the Rf value I estimate any drop or increase of my polar or non polar solvent fraction, but this doesn't provides an accurate measurement of solvent ratio inside the mixture. However if you are doing column chromatography, where the ratio of solvent doesn't need to be very accurate, you can try this method just for a quick look out for its polarity and thus estimate the solvent ratio inside the mixture. 

If you intent to recover a high purity of solvent (>90%) and if difference between boiling point of organic solvent used was small, fractional distillation was a better approach rather than using rotatory evaporation. Evaporated solvent having different boiling point will be condensed at different rate and thus the system can achieve separation of solvents from a mixture.  

Look out for azeotrope though as mentioned by Riaz A. Khan, you can't separate solvent composition from an azeotropic mixture using simple distillation. In another word you get what you are distilling. 

Example:

97% ethanol in water

Gas Chromatography (GC) using columns of different polarities would be the recommended technique to determine the composition of a solvent system. It is quick, simple, and cheap. You could also use HPLC as one colleague has suggested but it is not as easy and precise as GC. Before using the technique for determining the solvent composition, you should use it first with pure compounds to see where component comes out (retention time) and then use it with the mixture. To get good separation and ultimately good results, optimize the temperature program first. For more information and to know more about gas chromatography, you could consult with text books in the field. 

I do agree that the GC is the best option.

Dear M. Ammar Bader,

You can establish a table with your report of solvent at 20 ° C for example depending mixture density as ALCOHOLIMETRIC TABLES pharmacopoeia. So to check the title of distillate by using a table prepared beforehand.

So by densimeter you can estimate the percentage of your mixture.

Best regards

M. Ouhsaine Morocco.

Thank you very much for a person who downvote my answer which is a cheaper way to control ratio mixture. I ask you to explain me what is the error in my response.

B. regards

M. Ouhsaine Morocco