Here is a quick guide to see if your fork seal is leaking enough to be concerned. The visual guide from Ohlin's, a leading worldwide manufacture of suspension components, is concise and to the point. I wanted to keep this post quick but listed couple paragraphs from their guide. The below quoted statements also pertain to rebuilt or serviced forks and/or shocks. 

"Oil on the piston rod/fork leg/strut is normal and is there a lubrication to ensure performance" 

" Residue of oil and grease from the manufacturing processes of product can be hidden in cavities. with first usage and heat, the residue can become liquid and are sometimes visible on the outside of the product, this is normal" 

"Do not use strong detergent such as contact cleaner (brake cleaner) or equivalent while cleaning fork tubes. Do not spray water directly into the adjuster knobs and /or the ball joints."