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edited Apr 11 2010 at 0:33

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

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

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answered Apr 10 2010 at 15:47

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

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