Well I just got an old (1994) GS 500 E.
It was running on one cylinder when it went in for restriction, but was fine after a new set of plugs.
On Sunday I dropped the bike in my back garden when wheeling it on some gravel.
Not a scratch on the bike from that, but it took me a couple of minutes to figure out where to hold the bike to get it upright again.
After that the bike was running rough again, I think it's back to only being on one cylinder.

Anyone got any ideas what is up?

Sometimes when a bike goes over you can have fuel supply problems, you invariably loose some fluids from the drain hoses. Stick the fuel tap (petcock) on P (Prime) for a short while and try running it, if it's OK switch back to "ON" also try the obvious, check plug leads and plugs for damage.

Will try that, Thanks.

Any tips on how to tell if only one cylinder is firing?

Speaking from an unfortunate personal experience, your bike's condition isn't unusual.  One carb probably dumped into one cylinder and fouled the plug.  The best way I found out to correct this is to remove both plugs, clean them, and place the one that was wetter into the other cylinder, and vice versa.  Usually things wil clear up after a good ride.

OK running on 1 cyl... the exhaust will get hot only in that 1 cyl... the other will be much much cooler... might still be hot enough to burn you... but will be cooler than the other. Touch it at startup, and you can tell... after that sprinkle water on it... and it wont sizzle...
Cool.
Srinath.