Good advice so far.
Pull the rubber boot back and see how much lube is captured there.
Then remove the filler plug of the FD and look at the lube level. Don't move the bike or rotate the rear wheel or lube will be carried up on the crown gear and result in an apparent lower level.
Since you probably don't know what the level was at the time of the last FD lube change, this will only give you a base line and perhaps a suggestion that is it the FD if there is lots of lube in the rubber boot, and the level in the FD is noticeably low.
Then drain the FD lube and fill with a different color lube. It is worth looking at the FD drain plug magnet, but as noted by John in his post above, a leak at the input pinion seal is not associated with the metal generating crownwheel bearing failure.
I do have a slight suspicion that there may be an association with input pinion seal leaks and another form of FD problem, but it is only a suspicion. Since I started looking for the "wandering needle bearing race" described in this post: http://www.bmwlt.com/forums/showthre...pinion+bearing
I have seen more of them during final drive rebuilds. If the race is slipping off its seat, it isn't obvious or even easy to see unless you know what you are looking for.
I have replaced few FD input pinion seals, but have done many fewer of these than I have done of crownwheel bearing replacements. I recently accepted a FD to rebuild which had a leaking input pinion seal. A concomitant finding was that the needle bearing race had started to "wander" or "creep" off its seat. I don't understand what is causing the pinion needle bearing races to move the way they are, but I now wonder if there might be a relationship to leaking input seals; just my inquiring mind, no good data, or causal relationship to point to. Just two problems occurring in the same assembly at the same time.
If there is a relationship between pinion input seal failure and the slipping (wandering) pinion needle bearing race, that relationship would easily escape detection. Reason being, the input seal can be replaced without opening the drive or removing the input pinion assembly. To do so entails significantly more, seemingly unnecessary work.
I'm probably just pissing in the wind about a possible relationship between input pinion seal leaks and input pinion needle bearing race creep. But, if the input pinion seal needs replacing, in my opinion it is worth taking the cover off the drive and looking at the input pinion needle bearing race to check its position on the pinion shaft.
The more likely explanation of a leak at the pinion input seal is that it isn't the seal itself but rather the threads of the locking ring that holds the input pinion assembly in place. During assembly the input pinion seal is pressed into the locking ring, and then the locking ring is threaded and torqued into position in the FD housing. The threads of the locking ring must be carefully coated with Hylomar or comparable sealant, or FD lube is going to leak past the threads. It would be a natural assumption that a leak at the FD input pinion is a failure of the shaft seal, but in the few I have looked at, I've been more suspicious of an incomplete sealing of the locking ring threads.
addendum: oh, you might also just top off the tranny lube with what ever color lube is already in there, just in case it is the tranny output seal that is leaking. You probably could go a long time before the lube level becomes too low in the tranny but why take chances.