How to tell funk from disco
Funk and disco overlap broadly, but they are not the same thing. Funk lovers like me instinctively know what the difference is. But how do we know? One thing we could do is point to the beat. Disco uses that iconic four-to-the-floor pattern, and funk doesn't, so case closed, right? Well, it's not so simple. Four-on-the-floor is more common in funk than you would think, and while it's common in disco, it's not universal. So maybe you could point to production: disco is glossy and smooth, while funk is raw and edgy. This is better, but it's still too vague. The best way to distinguish funk from disco is to look at harmony. Specifically, we can look at how much a given song uses blues tonality. Disco uses all kinds of harmony, but funk is dominated by one specific kind: blues tonality. Disco might incorporate touches of blues, but blues tonality is the harmonic foundation of funk.
Let's look at a specific pair of examples: "Jungle Boogie" by Kool & The Gang and "Inside and Out" by the Bee Gees. "Jungle Boogie" is clearly funk, and "Inside and Out" is clearly disco. The difference is not in the beats; both of them use rock-solid funk grooves. Their harmony, however, is as different as night and day. The difference is in the harmony. "Jungle Boogie" is a one-chord groove all the way through, and its melodies are blues with some jazzy chromatic embellishment.
https://www.youtube.com/watch?v=-BM5wPOe0xQ
"Inside and Out" has a blues-inflected verse, but the rest of the song is modal or diatonic. That is why it's disco and not funk.
http://www.youtube.com/watch?v=kfwaz-NuUig