Using the thumb of your free hand, press into the gap created by the inner bend of the second knuckles. This test can help you determine how tight your fist currently is.  Make sure that you use the thumb and not the thumbnail. You should be unable to press into the gap with your thumb, but the effort should not cause any pain.  If you can break into the fist gap with your thumb, the fist is too loose. If pressing the fist causes considerable pain, the fist is too tight. A second test you can use to gauge the tightness of your fist requires you to gradually squeeze your fist tighter and tighter. Use this test to give yourself an idea of how a properly formed fist should feel.  Make a fist and place your thumb against the knuckles of your index and middle fingers. Squeeze your fist a little. The first two knuckles should tighten against each other, but the fist should still feel somewhat loose. This is the tightest your fist should feel as you strike with it. Continue squeezing your fist until the thumb reaches the knuckle of your ring finger. You should feel the first knuckle of your index finger weaken, and your little finger will squeeze inward in a manner that causes the knuckle to collapse inward. At this point, the structure of your fist is too distorted to be effective or safe to use while striking.

Summary:
Press into the gap. Slowly squeeze the fist.