There is no polite way to say that.

The polite thing to do would be to accept the coffee or not, without placing conditions on the host, or implying that their hospitality is possibly beneath your standards.

I wish I could upvote you more than once.

Ha, there's not. Instant is better (just) than no coffee, even though I wouldn't touch the stuff at home...

It's like if someone offers you a beer. Bud Light or a strong IPA may not be to your taste, but saying "what do you have" sounds greedy unless they've offered a choice. A free beer is still a free beer. You either turn down the offer or you are grateful for the free beer. There are no other options.

"What kind is it? I don't want to deprive you of a beer if I won't enjoy it." There, done.

I had a colleague who'd ask: "What kind of beer? Heineken? That's no beer."

After a couple of those, everybody knows your taste :D