For us sighted people, it's usually a combination of both sight and muscle memory. When I perform with a music sheet, I usually alternate between looking at my material and keys quickly in fractions of a second. It also depends if there are huge leaps, then I'll rely on my sight. Notes which are less than one octave to play usually don't pose any problem without looking.

Interestingly, when playing blindfolded, the playing in keys with flats and sharps becomes helpful because the black keys can be used as a guide. I'd say that playing blindfolded or without looking is actually a skill that many pianists should work on because for "on-the-spot" performances with little practice, it is really important to keep an eye on the music sheets and try not to get lost.

I play drums and bass guitar too and usually for drums, I have my eyes fixed on the material being familiar with the layout of my kit. However for bass guitar, I find it really hard not to look at the strings, because it's hard to feel the frets. And that's why I have so much respect for my blind violinist friend, with violin being a completely fretless instrument, it must mean he had practiced so much he has every millimeter of his instrument recognized by muscle memory.