get a syringe from the docrot that is one of them big plastic things for giving little kiddies medicine
they will be around the 10ml size although some can be as big as 25ml
turn your head upside down so it's flat (work it out with a spirit level on both planes)
put a bit of perspex over the top of one of the chambers and make sure there is a little hole in the perspex for you to drop bits of water or whatever you are going to use to measure out the volume of the thing
repeat this across the whle head to make sure that the head has a uniform and exact volume for each
then work out the volume of the cylinder when the piston is at the bottom of the stroke, bottom dead centre
now put the cylinder to top dead centre and measure it again
the first measurement plus the chamber volume of the head will be the total volume of the cylinder
the second one, with the piston at tdc will be the sweapt volume of the cylinder
and by comparing these 2 different vaules you will find the compression ratio
don't forget to add the thicnkness of the head gasket intot he equation
you need to find out it's total torqued down thickness rather than it's uncompressed thickness as this could be double what it's vaule is

and that's about it

unles you want to read stu's article which will tell you loads about others stuff as well as correcting the mistakes i may have made in this post