I measured mine with a large plastic syringe, a square piece of 1/4" plexiglas big enough to cover the whole chamber and then some, some thin lithium grease, and blue windshield washer fluid.
Drill a 1/4 or so hole in the plexiglas, such that when you lay it on the head over the chamber the hole is near the "top" edge of the chamber - you'll fill the chamber through this hole, and it also lets the air escape as you do so.
Put both valves in the head, and the spark plug - if the springs aren't on then use a bit of grease on the valve seats to make a seal.
Put a thin ring of grease on the head surface around the outside of the chamber so that when you lay the plexiglas on it, it makes a seal.
Orient the hole so it's "up" on the high side of the chamber, the plexiglas pressed flat against the head surface and sealed.
Use the syringe to squirt fluid through the hole until the chamber is full - carefully measureing how much fluid it takes as you go (the syringe I used wouldn't fill the chamber on one squirt, so I filled it to capacity (30 cc's) and dispensed fluid until it was down to 10 cc's, then refilled it)
I measured every chamber 3 times, and averaged the results in order to get better accuracy. Doing that will help "catch" any big mistakes and will help minimize the effects of any small ones. If you can measure 3 times and get results that are all within 1 cc of each other, then you know you're good (or you're making exactly the same mistake every time
It also helps to know what the heads "should" be. If Pontiac says they're 72 cc's but you carefully measure them to 70 or 75 over 3 times --- you're probably good. If you get 60 or 80 though, time to double check your procedure.
Go slow, be careful, think about what you're doing ---- you'll be fine.