It has been my experience that when head gaskets start to fail, they gradually get worse. Gasoline engine/diesel engine. Might last a while, who knows, 10 hours, 100 hours. Might get better but I wouldn't count on it.
It is also possible to look for combustion products and oil in the coolant which is another sign of trouble. When the problem is advanced enough there will be visible combustion gas exiting the coolant reservoir at which point you're hoping to make it to the dock
The question to you will be whether you think it will be worthwhile to proceed with the purchase
at this point given that there are known problems that could lead to an engine overhaul
within the first year. In most cases the answer will be "no" but if you're getting a fantastic deal on the vessel and have $20k in the bank for a repower
then perhaps it's worth it.
You could also try to start putting some of these costs on the seller i.e. ask them to pay for an engine survey
before proceeding with the sale