Separation of alkylated 1,2,4-triazole in solution
Joule and Mills Heterocyclic Chemistry [1] confirms that 1,2,4-triazoles alkylate with ~9:1 selectivity for N1 over N3, citing a paper in Tetrahedron Letters  from an industrial group at Merck as the source of the data[2]. 

The ratios reported were based on 1H-NMR of the crude product.[#] In this case, all of the compounds formed are known compounds, and as such it was easy to compare the data for 1-alkyl vs 3 alkyl. If the compounds formed were completely novel, it may be harder to tell which is which, though more complex NMR methods such as H-N or C-N correlation are often used in the pharmaceutical industry where this issue of regioselectivity often arises. 

As an aside, the paper points out that the N1 and N3 alkylated products are easily separable, in this case by distillation, but in normal cases, silica gel chromatography (or HPLC if needed) can usually be used to separate the different regioisomers, with sufficient effort! 



[1]: Mills, K.; Joule, J. A. Hetrocyclic Chemistry; Wiley:Chichester- Sussex, 2010 

[2]: Bulger, P. G.; Cottrell, I. F.; Cowden, C. J.; Davies, A. J., Dolling, U-H. Tetrahrdron Lett., 2000, 41, 1297  

[#]: The reaction was worked up, but not purified prior to working out the ratios. This is good practice, as it assures that you haven’t accidentally columned away some of the minor product, as is often the case