How to tell how many speeds a wheel is designed for
For Shimano and SRAM, at least:

8, 9 and 10 speed cassettes are designed to use the same freehub body. As higher speeds became available the sprockets were made closer together rather than the cassette getting wider. The freehub spline width is 34.95mm.

For road groupsets, the freehub body had to be made wider by 1.8mm (36.75mm) to accommodate 11 speed cassettes. 

For mountain groupsets, the 11 speed cassette fits on the narrower 10 speed freehub. This is possible because the larger low gear sprockets on a spider can overhang the inboard end of the freehub body.

Old or cheap 7 or less speed systems use an narrower freehub body.