It's usually somewhere near the shoulder, but it can vary. There should be lots of stuff stamped on the tank, and it can be a little tricky to decipher what is what unless you know what you're looking at. The hydro stamps are 5 digits, most often 2 numbers, followed by a letter or symbol, followed by 2 more numbers. The first 2 numbers are the month it was tested, the last two numbers are the year, and the letter or symbol indicates the testing facility. Sometimes the letter or symbol appears at the beginning or end of the sequence instead of in the middle, so a tank tested in November of 2006 could read 11A06, or A1106, or 1106A. If it's an older tank, there could be lots of hydro stamps on it.
Edited to add:
Some brand new tanks may not have had a hydro test yet, and therefore won't have a stamp.