How do you check for a blown intake gasket?

Look for leaking antifreeze/coolant. Usually you can see it leaking around the top of the engine. Also check your oil, if it looks milky it has been mixed with coolant and you intake manifold gasket is leaking. That being said, it is possible to have a leaking intake gasket with no sign of leaking antifreeze. Most intake manifold issues/leaks are coupled with poor engine performance and even sputtering and lack of power.