i just bought this pedal on ebay on a whim:
Modified Boss SD 1 Super Overdrive Guitar Effects Pedal PD 5988 | eBay
and it says that it has an unknown mod but its been modded. how would i tell what the mod is? or what has been modded? do i just go on youtube and compare sounds with stock ones? or are there specific things that i can look for?
There's no way I'd be able to identify a mod by sound alone. If you can, you're a better man than I.
Best bet is probably to get a shot of its guts. Some of the electronics nerds here might be able to identify it from that.
If it's had a flux capacitor soldered on, it's an Emmett Brown mod. Very sought after.