how to tell if head gasket is gone?

[Gawri20]

foolishly bought my first car last month, not knowing anything and in a rush. too many problems with it, head gasket gone, crack on the head, starter motor dead..you get the drift.

canceling insurance is too expensive so im just putting another car on the policy and getting rid of this one.

so as i started looking for an new car, i was told that if there is yellow gunk on the oil cap (like my car) then the head gasket is gone. but im also aware that it could just be a condensed mixture of oil and water (or something along those lines)

my question is how do i tell the difference, if an oil cap has gunk on it, whether its perfectly normal or a blown head gasket.

5 of the cars ive gone to see already all had this yellow gunk in them.

thanks