You can try a colorimetric method, such as phenol sulfuric acid as in Dubois M, Gilles K, Hamilton J, Rebers P, Smith F. 1956. Colorimetric method for the determination of sugars and related substances. Anal Chem, 28: 350 - 356. 

You can try HPLC,

Mobile phase : water

Column : Jordi Gel Glucose Mixed Bed Guard, 50x10mm, 5μm and, Jordi Glucose DVB Mixed Bed, 250x10 mm, 5μm 

Refractive Index Detector

good look.

Dears Trevor and Zakaria;

Thank you so much for your suggestions. My goal is not indentify, but only detect/quantify the total polysaccharides that may remain soluble in the 80/20 hydroalcoholic solution, so column chromatograpy could be too complicated.

On the other hand, TLC may be an option...

I'll check for conditions.

Thanks again!

If you change your mind about HPLC you can also try Zakaria's solution but detecting with an ELSD detector and amine reverse phase column.

The colorimetric method -  the best and simple from all advising.

Yeah go with a colorimetric test, Marc's suggestion might work but if it doesn't you can try this one which I have used for polysaccahrides before: It's called Periodic-Acid Schiff (PAS) stain technique -

1) Add Periodic acid to 5% w/v and leave for 5 minutes at RT.

2) Add "Schiff's Reagent" and leave for 15 minutes at RT.

The solution should turn magenta/pink if there are carbohydrates present.

You could probably make a calibration curve with some polysaccharide standards and use UV-vis spectrometry to make it quantitative.

Try the following procedure : take an aliquot of your 70:30 ethanol-water solution, and evaporate it in the rotavapor, to complete dry. Then, if you want only to detect the presence of polysaccharides (not quantify), you can use the phenol-sulphuric method (Dubois et al., 1956), like Mark Lahaye suggest. But if you want to quantify the monomeric sugars, if any polysaccharide is present, then you must hydrolyse the polysaccharides in individual sugars, by adding 2M trifluoroacetic acid, then heat for 2 hours at 120 ºC, then you can detect yours monomers by, e.g., HPLC, or derivate it (acetylation) and quantify them by gas-chromatography.

Thank you all for your suggestions.

I will start trying the colorimetric alternatives (Phenol-sulfuric / PAS). If any of these do not work I will move to HPLC. I will also check for the TLC, for a qualitative, simple yes/no answer.