Effects Loop Mod Question

[FlyingPigPhotography]

I have a Lab Series L3 amp, that I got used. It originally had a TRS Effects loop, but my amp has TWO jacks in that area (obviously some kind of modification). I want to try to determine what has been done in order to use the mod (preferably without going inside the amp). My thought is that they may have turned the TRS jack into two 1/4" Effects Loop - Input and Output Jacks. If so, How can I test (either using pedals, headphones, or a multi-meter). Thanks a lot for your help : )

[Jazz P Bass]

You can use a volt meter set to read Volts ac.
The send jack will have a signal on it.
The return jack should drive the power amp if a signal is inserted.

[Mick Bailey]

Plug into either jack with a regular cable and touch the tip of the lead. If the amp buzzes that's the return, if not it's the send.

[FlyingPigPhotography]

Thanks guys, I will try the head first, less technical : ) Let it never be said that guitarists never use their head!