working out compression ratios

[totta Crolla]

Apart from physically being able to achieve the actual ratio required, 4age's are not normally a problem, the limiting factor to compression ratio is generally the fuel octane and the ability of the ecu to control knock. Knock control is really just a failsafe and should only really be utilised when bad fuel is used.
As far as actually measuring, yes the piston should be installed in the block with the rings fitted and it should be connected to the crank.
Most piston manufacturers will tell you the specs. of the piston including any difference to the standard piston, raised piston crown cc etc.