I assume you do not have a compression testing gauge, so try this..
Remove the plug.
Get a small rag and stuff some of it into the spark plug hole nice and tight. Then kick the engine over like you're gonna start up.
The rag might pop out or it might not. If the rag doesn't pop out you've probably got a serious lack of compression problem.
Compression can leak past the rings or through a hole or gap somewhere above the rings... like through a crack in the head gasket or through an open decompression valve (if the engine has a decompression valve) or somewhere else above the rings. Or there could be a hole right though the piston crown.. you get the idea..