Sail cloth is like any cloth of the same material. Stain removal begins with finding out what the stain is from. Any idea?
These days sails are typically made from a special weave of Polyester and coated with a resin to lock the weave from premature stretching.
If your sail dates back to the late 1950s or is a low quality as found on some styrofoam children's boats, it might be made from Nylon. Nylon can be severely weakened by chlorine bleach so should your sail fits these categories exercise caution with what you try.
Here are a couple of sail maker's tips for sail cleaning...
If the sail is still coated with sizing ( slick coating applied when sail cloth is manufactured) then lay it on your lawn, scrub with a soft brush, cool water and mild soap, then rinse and hang dry. If the sail feels like an old t-shirt then you might be able to remove surfrace dirt by washing as above, however stains will likely remain. In the event there are rust stains try Oxalic acid. Bottom line, the older the sails the harder they are to clean.
If it is mold you can throw it in a pool. I am not joking, the North Sails guy I know told me straight. The chlorine shocks and kills the mold. I use a mild mix of clorox and water, and I mean MILD...especially with Kevlar or Carbon sails. I have a Pentex set that the main has a nice brown spot. I can't seem to get it off, so I dubbed it 'the brown spot of awesomeness'. Good luck.