Agree that you need a competent survey for peace of mind (and to get insurance), but you can do your own prescreening with a small plastic hammer or even a small screwdriver handle.
Tapping around the suspect area you should hear a sharp, solid "Thunk" if the deck is sound. A delaminated or softened area will show up as a definite hollow "thud". Once you find an example it's easy to tell the difference. Another place you can often find some delam is around the cabintop halyard winches (due to the stress of hoisting sails and the torsional forces from the cranking of the handle). This is especially true if the winches were installed after the fact by an owner moving them off the mast.
Back to your mast step, if the crazing is not deep, there is no apparent deflection of the area of the mast step (assuming deck stepped), and it sounds out OK then it's worth persuing and getting a more professional assessment. This is a high stress area and gelcoat crazing is not unusual in older boats. If it sounds out like there's nothing but "porridge" between deck skins, walk away....