White foam is a sign of water, but there are several ways for moisture to get into the crankcase. If you are concerned about a blown head gasket, test it with this.
Combustion Leak Tester
It tests your coolant (while in the radiator still) for combustion gases (CO2, etc) which are indicative of a blown HG. If you use this, make sure you follow the directions exactly.
I'd drive it for a while and see what happened, or drain my oil and check for moisture in it.