Best way to tell is to drain a little oil after the car has cooled. Oil floats and this at the drain lug the water will come out first - usually white/milky. If not, then as said above.

The gasket for the cap can be ordered. I just put a new one on myself and it seals much better.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car