What am I looking for?
Angle: The back of the bridge (the side towards the tailpiece) should be perpendicular to the top of the violin. This means that the front of the bridge will seem to lean away from the fingerboard slightly. If the bridge leans towards the fingerboard, the feet are not making proper contact with the instrument and the sound will suffer. You can correct a leaning bridge by *gently* pulling the top of the bridge back towards the tailpiece while stabilizing the bottom of the bridge. Ask your luthier to demonstrate this technique.
While tuning, watch the bridge and keep it upright. The top of the bridge will be pulled towards the pegs whenever you tighten the pegs. The effect is a cumulative warping of the bridge. Eventually the bridge may even fall down. Using a pencil to lubricate the grooves on the bridge will help minimize the pull of the strings on the bridge.
Position: The placement of the bridge determines the vibrating string length. In most cases, the proper position will be on a line between the two inside notches of the ff-holes. However, some violins have different proportions, so that rule may not apply. Check with your luthier for the proper placement on your instrument.
Condition: Bridges may warp or crack, or they may slump so that the eyes are no longer open. In most cases, this means that the bridge should be replaced. The E-string may also be cutting into the bridge, which can be fixed by reinforcing the bridge with a piece of parchment.
Height and curvature: Look down the fingerboard at the bridge. The E-string should be very close to the fingerboard, while the G-string should have a bit less than twice the clearance. The curve of the bridge should be very close to the curve of the fingerboard. If this is not the case, chances are the ‘action’ or ‘feel’ of the strings will be wrong. If the bridge had been correct at some earlier time, it is likely that the fingerboard has worn and needs planing, or even that the neck needs to be reset. Consult your luthier.