I noticed my left side fork seal was leaking. Some residue was noticeable down from the leak, so I am not sure how long it was going on, but I don't recall seeing anything to cause concern a few weeks ago when I last washed the bike.
I cleaned and refit the existing seal, and it seems to be fine now. a few rides later, and no signs of leak.

When I was "repairing" the seal, there was oil coming out, so I assume it still had a decent level, but how can I tell?

My only concern now, is not knowing how long the leak had been going on, and not knowing how much fluid may have leaked out. Is there a way I can "top off" the fluid without removing everything? How would I do that? How would I know how much to add?
Anyone know any cool tricks I am not smart enough to figure out on my own?