Is there a kit or product I can purchase to test if there is oil leaking into my coolant, or vice versa? I'm trying to determine if my head gasket is blown on a 98 e39 528i that's overheating when Idle. I've already gone through the entire checklist of things to check and everything checks out fine except I can't test the clutch fan properly because it's 20 degrees in NY and even with the A/C on I can't get the aux fan to kick in, but I don't think that's my problem anyway, not at these outside temp.

Your auto part store should have a kit that test for exhaust gases in your antifreeze. Cost around $60. Check your oil for presence of water too.