You don't say what keyboard you have in mind...
In recent arranger keyboard there's a level meter in Master/Compressor settings, which shows total (L+R) level.
If you wish to see L/R separately, then you need USB Audio Interface (if your keyboards doesn't have it built-in), so you can send audio signal to PC. Now you can use any audio recording software of your choice (most have so called peak/level meters). Btw. you don't need to record anything: audio level is also shown in record stand-by mode.
PS. Just saw your signature says Genos and Pa5X... means, in Genos case, you need audio interface (didn't check if Pa5X has audio interface).
I hope that was of some help,
Bogdan