if the piston has a bowl in the centre (i.e. low comp) then it's feasable you could measure the top of piston through spark plug hole...compare it to standard and if it's a longer distance it's conceivable that the piston bowl is lower meaning lower comp than std.

You could also whip the sump off and have a look at the underside of the pistons, if they say a manufacturer or codes on them it might shed some light to the situation.

Good luck
Dan