The in game market history is an awesome tool to see when there are peaks for demand in certain goods, which presumably correspond to times when lots of people play (e.g. Evening in North America.)

By looking at tech purchases, I can workout timed when lots of casher are online, although this is a bit uncertain due to standing orders.

Surprisingly, the times when lots of casher play isn't the same as when lots of Indies play.

My question is this: how can I work out when the most techers are online?

You can't, unless you have a market feed from EE that sends information on when goods appear on the market - and these only notify you if something appeared at the same/cheaper price. The tech could have been on the market for hours for all you know.

Plus every strategy buys and uses Res/Bus techs, not just cashers.