Yeah, good old doppler. And my age is posted, heh heh.
Even better than the pitch/power rules-of-thumb is an inertially- based flight path marker with a HUD. Speed is still a problem if you are close to the mach limit. Less so if close to stall, as AoA indicators are really great for that. In fact, the AoA can be used at cruise speed, but it is very senstive, like fractions of a degree for a few knots.
BTW, my first inertial system was supplemented by a doppler ( nowadays they use GPS, but still use inertial vectors for accuracy in the tenths of a knot or even less). We could even do INS alignments inflight using the doppler and then providing a nav fix. This was late sixties/early seventies. You can do the same with the GPS.
Chris has a good idea about an old-fashioned anemometer, but the lazer doofer seems to offer a good backup for the hole-in-a-pipe things we use now.