Using the temperature and pressure conditions of the beer at equilibrium conditions and reading the volumes directly from a chart easily obtain determining the volumes of CO2 in beer. Equilibrium means the same amount of CO2 is diffusing out of the beer as is being dissolved back into solution. It is critical that the readings used for determining CO2 volumes are taken under equilibrium conditions and the instruments used are accurate.
The Patient Method
1) Clean and sanitize your keg thoroughly and connect your gas line to the black liquid out disconnect. Pressurize your keg to 10 psi, wait until you hear/feel the gas stop flowing, disconnect the fitting from the keg and release the pressure from the valve on the lid or through the gray gas in disconnect. By doing this you're purging the oxygen out of your keg.
2) Gently rack your beer into the keg just as you'd rack it into your secondary fermenter or bottling bucket.
3) Replace the lid on the keg and repressurize again to 10 psi, let it sit for a minute, bleed the pressure off again to re-purge (also known as "burping" your keg).
4) Determine the temperature that your beer will be during carbonation and set your regulator accordingly (again, refer to the chart). EXAMPLE: You just kegged your Willamette Valley Golden Ale and you need the carbonation to be just perfect to present that wonderful hop aroma to your nose without making you feel gassed up half way through your first pint. A factor of 2.4 volumes is a really good number here (your results may vary - I don't like my beer to be too gassy). Your fridge that you'll be dispensing from keeps a fairly constant 40 degrees F. You'll want to place your keg in your fridge with your regulator set at just over 11 psi (you see it on the chart, right?). Give your beer 48 hours to carbonate. It will reach its saturation point within this amount of time and the regulator will shut down altogether. Remember that the gas should be connected to the black beverage disconnect so that the CO2 bubbles up through the beer.
The Impatient Method
1) Follow step 1 through 3 from the patient method.
2) Set your regulator to its highest setting or 60 psi (whatever comes first), pressurize your keg through the black beverage fitting until you hear/feel the flow of gas stop, disconnect the gas (this is important-you don't want beer flowing into your gas line) and shake your keg vigorously for 5 minutes.
3) Repeat step 2 until:
a) your beer will receive no more carbonation at this pressure setting.
b) Your testicles drop down to your ankles.
c) You die of a massive heart attack.
Obviously this method should only be used as a last resort. Even if it doesn't cause you grievous bodily harm it leads to rough handling of your precious homebrew and uncertain carbonation levels.