Put it under water... see where bubbles are coming from. Seal the heck out of those areas with silicone.
Or
Have your mast in the shade on a hot day. Put it in the sun... soap it up. Seal the smack out of where it is bubbling.
or
look at every single fitting on your mast and if there is a question smear the hell out of it with silicone.
or
hobie bob it

One of the last times I went out sailing I capsized and there was so much wind on the bottom of the tramp it was pushing the mast under and once the sails were in and it was drifiting it pushed it down more. I was really worried about turtling there although the bay were I was was probably not deep enough to go completely turle. That would lead to a problem with damage to the mast if it did hit the bottom. The mast came back up easily. I haven't looked specificly for holes in my mast byt putting it under water, etc... but I did look around soecificaly for issues and goobered areas up that I was a little concerned about. That being said... I think there is some styrofoam in my mast though.