how can you tell if you have a blown head gasket?

Compression test , but you pretty much described loss of compression by saying " it's like it don't have no resistance". The other thing that would make the engine turn over/crank with little or less than normal 'resistance' , is a broken or stripped timing belt.

Consistent loss of coolant, and overheat can be the first indication, but a real pressure test and hydrocarbon test should be done. But, I agree with ziptie12, you may have a broken t-belt. We just did one a few months ago. we can check it for you, if you call and schedule. tr 509-545-6165