Is there an easy way to tell if a motorcycle is negative earth or positive earth?

Look at whether the + or the - battery terminal is attached to the frame. Alternatively, check with a meter which battery terminal has voltage relative to the frame. The one that doesn't is ground.

If there is no battery fitted, it can be quite difficult.

What bike we talking about ? - alternator and generator bikes have different ways of figuring this out.

And only voltage regulators and rectifiers and diode blocks and zenors and ammeters (and the battery) are fussy about this, the rest doesn't actually care - as long as everything is in synch.