put the engine at TDC (well, just make sure one of the pistons is as high as it can go)

and pour some liquid in there (i think people use parafin, cant remembe tho to be honest) till its completely full, then use a burette (sucker thingie you get ijnjections with) and measure how much liquid there is there

you can then use this information to work out the compression ratio of the engine

OR the quick easy way... use a ruler and measure how much space is left when the piston is at the very top.

TBH your tuner that fitting the engne should/will do this all for you, as even a stupid spannermonkey mechanic wouldnt be stupid enough to put an unknown spec engine back in a car without checking what it actually is! lol