I think it's unlikely to be the OT or recto tube since the amp had worked before (at least one channel).
It might be a wrong or missing wiring. Could you post some pics of your build?
To measure your voltages put the black probe of the meter to ground and the red to the spot you want to measure.
Set the meter right (AC, DC, OHMS).
Be aware, the voltages in tube amps can KILL you.