Two things to try;
1) Dust some talcum powder around the suspected area, and any concentrations will darken the powder. 'Not real reliable, though, as it tends to cling to anything that has ever had an oil mist on it (pretty much everything on my bike).
2) Obtain some UV dye from your local auto parts store, dump a capful into your oil tank, run for a couple of minutes, roll it into a darkened room (your garage will usually suffice, but, the darker, the better), then train your '70s black light across all questionable regions. Any leaks will show as bright green! If the leak is small, you may need to come back and revisit after putting on some miles.
Nathan