I think if you pull the fuel pump you can feel for the slop in the chain. Does this maybe have the phenolic top sprocket? If steel, there's very little chance that it's slipped.

Quickest way, is that IF you know where the timing is supposed to be, and it's way off, it's slipped.

Take a wrench and rotate the engine cw/ ccw against the chain "slop." If it's 10* movement or more, I'd be suspecting slippage.