There is one answer in the following paper, along with a nice bibliography of other techniques:

[MR1343700 (96k:20100)][1]
Hsu, Tim(1-PRIN)
Identifying congruence subgroups of the modular group. (English summary)
Proc. Amer. Math. Soc. 124 (1996), no. 5, 1351--1359. 


  [1]: http://www.ams.org/mathscinet-getitem?mr=1343700