So I've read that
いかがですか means "How about it?" and "How is it?". So if I wanted to say, "How's the coffee?" would it be コーヒーはどうですか？ or コーヒーはいかがですか？
Also is there an "informal" way of saying いかがです?
いかがですか is a more formal way of saying どうですか, and similar to どうされますか. Saying コーヒーはいかがですか can also mean "Would you like some coffee?" Context should clarify it of course, but I think that どうですか would be preferred for asking about the coffee, and if you really wanted to drive the point home you can say something like コーヒーの味はどうですか？
To make it more casual you can just use どう, as in コーヒーはどう？