A lot of computers have a rear-wheel sensor option, longer cable for wired type. I got one one for an old Cateye for riding my trainer (not rollers, which work fine front-mount). I suspect that digital wireless systems should be front or rear-sensor compatible. You can always contact mfrs to find out.

You can also try GPS, which has a little lag, and sometimes under-records your maximum speed (if you don't hold it long enough), but it's pretty good.