how do i tell that my injectors are leaking?
 

as above?

apart from fuel smell in the engine bay and crap mpg!

I know it doesn't answer your question directly, but why not just put in new o-rings regardless to rule it out? There is an old post with the part numbers somewhere around. IIRC all the orings will run you <$20 so it's pretty cheap insurance if you are not sure.

I think you found it. Send them out for testing.

I found mine buy pressurizing the intake manifold (when I had a supercharger in the car). I poured soapy water everywhere and at least two injectors were blowing bubbles.

Usually you'll experience crappy starting too as they bleed the fuel pressure out of the lines they'll cause long crank times and can flood the engine a little.

Changing the O-rings is a good start but hell it's cheap to have them rebuilt and flow matched.