Yes, NMR assay is commonly used for  assay determination. The NMR not only will give you structure of your compound but also the purity (assay). You need to use some internal standard for determination of assay of your compounds. Using reference proton from your internal standard you can establish the assay . It much simpler than GC or HPLC where you need to do a method developments etc. 

As AM above says NMR can give structure and quantity of substance being analysed, but you will need help. % Purity is easily measured by 1H NMR provided you have access to a working spectrometer of field strength >40MHz.  An excellent summary of how to proceed can be found in a general review see Trends in Analytical Chemistry, 2012, 35,5, for methods, developments and applications. Note that it is also possible to use the residual solvent peak as an internal standard without adding another standard. see Pierens, J. Nat. Prod. 2008 ,71, 810–813. Both papers are available on the web as pdf's.

Sure you can determine MeOH concentration by NMR, especially from non-aqueous solutions. The question is at what concentration level. With GC you can go down to several ppm level, while with NMR, 1 % can be a problem, especially from complex mixtures and in case of overlapping signals.

There is no standard procedure at present but results in much higher possibility to estimation of toxicological effects of intoxication and molecular-renal failure at concret part of kidney.

NMR has been established for the routine analysis of methanol for some years. For example, methanol is included in the commercial Bruker WineScreener.

If you need high sensitivity, such as for analyzing alcoholic beverages, it might be important to suppress not only the signals of water but also of ethanol. You then get detection limits as low as 0.02 g/hl of pure alcohol (the maximum limit for methanol in vodka in the EU is 10 g/hl of pure alcohol, for comparison).

Article Application of automated eightfold suppression of water and ...

The presence of methanol can be confirmed by 1H NMR ( singlet, ~3.3ppm, but does vary with solvent +/- 0.2 ppm, see Fulmer et al Organometallics, 2010, ,29, 2176), and further by its accompanying 13C satellites if visible ( 1J~142Hz). If the spectrum is congested we run HSQC experiments which can help define the methanol. We also found that 17O is really useful nucleus since it is well separated from water (0 ppm) and from ethanol(~ +8ppm) with methanol at -33ppm.

Thank you for all respond...I'm very appreciate it...Actually, I already quantify the methanol concentration using GC but the peak is too small and the results is insignificant for various samples and hard to me to analyse. So, I want to try using NMR but I'm not sure at what level the NMR can be used in to measure the lowest concentration of methanol.