Only if the frequency is different, which I doubt as they are all the same model. What I need is a way of detecting if they're flashing together.

There are two possibilities.
1) The LEDS flicker at the line rate.
In this case they will probably all be in sync.


The LEDs flicker at the PSU switching rate.
In this case they will NOT be in sync. Switching power supplies are usually free running and the frequency will be drifting around. Even if they are the same model, component tolerances and temperature variations will mean they are on slightly different frequencies and there will be no defined phase relationship.

mark