Not to detract in any way from above advice about knowing power and attitude settings, and agreeing that one should be able to fly sufficiently accurately without ASI or most other instruments.
But nevertheless, if you fly into wind (of course, all competent pilots know how to determine that direction if the wind is significant), and note GPS speed; then downwind at the same airspeed/settings; and note the new GPS speed; the difference between the two is twice the wind speed at that altitude, and GPS speed into wind plus wind speed is your true airspeed. At close to ISA, that is close to IAS, otherwise corrections need to be allowed for (beyond my capability for mental arithmetic as I don’t know the correction factors off by heart!).
Trying it might make a useful exercise sometime when you have nothing better to do while bimbling about.
IMHO.
Chris N.