i cant tell you straight out of the book or anything. but i live in richmond and carry a 3 1/2 blade and all the police ive encountered have been cool. but also i look normal and dont act like a fool generally. ive had a few comment that a folder clipped in the pocket is concealed and others say that the clip being exposed makes it not concealed. carry a 3 inch sheath knife too and never had any kind of problem.
so as far as i know: nothing over 3in folder, belt wear ok, no auto knives, or knives with a d-gaurd like trench knives.
but like i said in my experience, just be cool and leo's will be cool