Can you be sued if there is no trespassing sign on your property?

You can always be sued, but generally the only duty you owe to a trespasser is to not intentionally harm him or her. Unless you knew of a dangerous condition and knew the trespasser was there but did nothing to warn him or her, you probably aren't liable. However, if the trespasser is a child and you had something on your land that would draw kids like him or her (a junk car, a pool, etc.), you can be liable under "attractive nuisance" theory.