Have a good look around/underneath the engine for coolant leaks. Also does it smell like the engine is burning coolant at the exhaust tailpipe?
You could also try draining the coolant into a clear container or something you can look at the coolant with and see if there is any oil mixture. Then refill the coolant with fresh to the mark on the water bottle and make a note. It could just be the crack letting in moisture. If it is just a little leak a bit of radweld might seal it.
Another option is to get an oil & filter change and see what that comes out like. If that is all mayo then it confirms it - theres problems. If not good good etc.
Couple of cheap options to start with before doing any major work.