It will be easy to see the really big ones, but the smaller ones will be harder to identify in the
water. The big
blisters will frequently weep when the
hull is out of the water and drying out.
If I was concerned about possible blisters, I would
haul out the yacht and have a look before I purchased the yacht. If I was concerned about blisters on my own vessel, I would just wait until the next haulout to get a definitive answer to the blister question.