Converting two prong to three prong outlets, are the individual boxes usually grounded in the 1920's construction era?

Nope... assuming the house has its original wiring. If it does, you should seriously consider having it rewired if only for safety reasons.

I am not saying this to be a smart alec but if you have to ask then it is likely you should hire this job out. To know if a box is grounded is a very, very simple test. Put you test lead on the hot wire and the other lead on the box. I'm with Smoothy and think a total rewire should be considered.

Yep, me too. You may even have Knob and Tube wiring at that date and they didn't usually run ground wires then.

Actually it is not that simple checking for ground. You will get a ground reading even if old BX cable was used. Old BX, without the thin aluminum bond strip, cannot be used as an equipment grounding means!