Check outside for a posted menu. You don't want any major surprises, even though the "best" places won't likely post menus. The "worst" places also won't, and if it's a "best" place you'll already likely have heard of it. (Those tales about unsuspecting travelers in various places in Middle/Eastern Europe being billed for $1K each and beaten if they refuse to pay are too prevalent to ignore entirely.)

After checking for a posted menu, look inside and see if the restaurant is crowded or empty. If empty, walk on. If crowded, check to see the expressions of the people when they come out. Do the people look happy or vaguely dissatisfied and frowning?

If NO people are coming out, this is either a very good sign or else a very bad sign. . . .