Most digital tachs read light pulses/time=frequency=RPM
They will all react to the strongest pulsed source.
If you have flourecent lights, they will read 3600 in the US
You need a steady state light for the prop to interrupt, like the sun.
I haven't tried them indoors but you may try one of those very bright LED's flashlights
The light source is continuous and might trigger your tach properly.