Not sure what brings up the question. But STSS is right, the block test kit is the BEST way to tell. However you'll also have various symptoms depending on the progression of the problem. In the early stages the most notable symptom will be inexplicable coolant loss. First every couple of months, then every few weeks, and then every few days. Sweet smelling steam from the tailpipes is another sign, and finally overheating. First under loads, and eventually all the time.
Check you oil cooler in the radiator if you have one, thats the most likely place for oil to enter the water, and vis versa. I think the only way to get water into your oil short of a really gross head gasket failure (which would have your exhaust pipes puffing white smoke like a freight train) is to have a cracked block, and/or head. If its not puffing smoke, and overheating its probably not coming from a head gasket problem.
The engine may still be OK. Im sure you all have seen the thread where I posted the pics of almost pure coolant in the engine. Amazingly the bearings seem to stay good in these, even with that milkshake mix. But stop driving it, stop running it, stop doing anything with it and fix it ASAP.
The last one that I worked on had a cracked block. Same milkshake mix. It also smoked white (coolant) like you would not believe.
Don't give up on it just because of the milkshake mix. If you don't plan on fixing it right away, then drain the oil & coolant before you start it again. Fill the engine with about 7 quarts of good quality oil, and have it running for a couple of minutes until this fresh oil cycles around, but before the engine has a chance to heat up completely. Remember, now you have drained the cooling system.