If you used the tape as suggested there is not too much to sand. I would try to clean the excess epoxy off but don't sand thru the vaneer.

Dan 

Ideally, you want to sand only the epoxy, leave all the wood alone.  That's if everything align's just perfectly.  A lot of builders will use some type of scraper to trim as much of the excess epoxy as possible.  If you don't have a scraper, use a large, flat sanding block and be careful of sanding into the ply.  You can support the underside of the scarf with several layers of wide tape or plastic and hold down the panels away from the scarf.  That will raise the scarf's joint up a little so you tend to sand more epoxy than plywood.  Don't worry about removing all the epoxy; it just needs to be fairly smooth.  Later coats of epoxy will make all the wood look the same epoxy-stained color, unless you have sanded into the next layer of ply.  Be most careful with the outside surfaces; there're the ones that will be seen.  And you can sand them more when you are preparing to glass the hull.  Good luck.

I prefer scraping as "ootdb" mentioned above, better control and you minimize removing the wood on either side of the glue line. Placing packaging tape on either side of the joint prior to gluing will help cleanup from squeeze out when dry.

On my first boat, a SOF, I had had two very visible scarf joints that I sanded and sanded and sanded... finally gave up accepting the "stain" that was the extra epoxy (thickened with cab-o-sil) and was pleasantly surprised when I epoxied the whole piece that the smudge that I could not sand out disappeared.  I am anticipating a small "line" on my WD12, scarfs did not go as well on this project.  Still should look good.