How can I find the value...?

It's going to depend on if there is a letter following the 1934 (and what that letter is). Because of the way currency is dated, Series 1934 bills were first printed in 1934 through 1950. Series 1934 were printed in 1934 and 1935, 34A was printed from 1935-1945, 34B in 1945 and 1946, 34C from 1946-1949, and 34D from 1949 through about 1951. Those are only rough dates--it wasn't uncommon in those days to use "old" printing plates (say Series 1934) and keep using them even after a new series had begun, so even though a 1934 might not get printed until the early 1940's, when they were technically issuing 1934A's.

If it's worn about like the average bill you'd get in change, most of the different Series 1934 notes are only going to be worth a couple of dollar over face value. They just printed too many of them for them to be worth much of a premium.

If yours has a star at the end of the serial number, that means it was a replacement for a note that was damaged during printing. In that same "average circulated" condition, you could probably sell it for between $40 and $60, depending on the series.

Also, there were some Series 1934 and 1934A notes that were specially printed for use in Hawaii immediately before and during WW2. Those have brown seals and serial numbers, and say "HAWAII" on them in several places. Those would generally sell for $60 to $80.