I agree with the comments above about oil/fluid leaks being a pain to find. Also second the request from dolt about model/year.
Sometimes when the oil filter is changed, oil can spill onto the front of the engine and lay around for a while until showing up. Try this:
I suggest that you clean the engine and tranny very well with soap (detergent) and water. Use a soft bristle, non-synthetic scrub brush and get the underside and everything you can reach as clean and oil free as possible. If there is a bunch of oil and road stuff on the engine and tranny (especially on the bottom), you might try getting some spray degreaser from one of the auto parts stores and hit the bad stuff with that.
Once you have the engine and tranny as clean as you can get them, dry them with a towel and let them sit out in the sun for a while. When good and dry, then take the bike for a pretty long ride and then park it over some packing paper or cardboard. Inspect the engine and tranny especially underneath, between the inner primary and the tranny, and around the valve covers. Check around the oil filter too.
If you can't see anything, let it sit over the paper/cardboard over night and then check. If still not obvious, then start riding and parking the bike over the same spot on the paper/cardboard until you see some tattle tails. Once you see evidence, start a real search for the source of the leak.
Good luck on your hunt!
Cheers,
TQ