Yep, visual, running with the engine cold or maybe even better to jumper the fuel pump relay so you don't need to run the engine and when you find a leak wipe it and watch to make really sure it's only the o-ring leaking and not the injector itself.

__________________
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)