If you think your bellows might be leaking, there are a few tests you can do to check.
One way is to hold the bellows up to your ear and listen for any air escaping. If there's a leak, you should be able to hear it. Another way is to put your hand over the end of the bellows while they're closed and squeeze them slightly - if there's a leak, you'll feel air coming out around your hand.
The most reliable way to test for a leak is to use a piece of paper. Hold the bellows up to the paper so that the paper is touching both sides of the seam where the two halves of the bellows meet. Then close the bellows and squeeze them slightly - if there's a leak, you'll see the paper being blown away from one side or another.