The proper way to know the source of the leak is to degrease the area. Then,
1. Add a leak detector to your oil. I once got one from the dealer. Drive the car for a day or so depending on how severe the leak and then use black light in the dark and it will shine green (flurosent).
2. There is a powder that you can use to detect an oil leak. It is available at the dealer. I don't know the P/N, however, the dealer will know what you want.

Any other way would be a waste of time and money.

I had a similar leak in my 190e on the same side as yours but between cylinder 3 nad 4. It was the head gasket. I replaced it and all my oil leaks are gone. So far, 2200 miles after rebuild and still full and clear as comapred to a quart every 400 miles.
Good luck

PS. Some part stores may have similar items (do a search on the web)

__________________
Meza
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer