How can I tell what rate my stamps are when they just read first class?

Then they're good for first class mail no matter what. The U.S. post office has started a policy of honoring first class stamps even through future price increases.

The rate would always be the current rate of a first class letter and that is now, $ .41. When the Postal Service has a rate change, they have often issued initial stamps with no dollar value imprinted or assigned. That slowly changes until they run out of the first issues. No other denomination has "First Class" written on it.

They're worth whatever the current rate is.