Lots of good points here. If its an intake gasket as mguay says, you wouldn't want to pull the heads. Im not sure of all the years and models affected but GM did have lots of intake problems. The best test to do would be a leak down test.
By doing this it will also give you an idea of the piston ring wear and other issues. A leak down is better than a compression test and if done properly might better give an overall condition of the engine. You would be able to determine the condition of cylinders even if they are not the ones with blown head gasket or cracks. That may steer you to a new engine rather than invest in one with more wear than you would like.
