Afraid not, it will always read 0, nothing else.
On the brand new A320 series I fly now, checking the difference (and there is one - a knot or two after a couple of hours) is part of the shut-down checklist to determine if it is necessary to re-align the IDIRS, or even to report them in the Tech-Log.
Originally Posted by shlittlenellie (post #19, above)
The systems manuals for the 737 state that there's a barometric input to the FPV:
The Flight Path Vector (FPV) symbol represents airplane flight path angle vertically and drift angle laterally. The flight path vector is displayed on the PFD when the EFIS control panel FPV switch is selected on. The FPV shows the Flight Path Angle (FPA) above or below the horizon line and drift angle left or right of the pitch scale's center. The FPA uses inertial and barometric altitude inputs. The vertical FPA is unreliable with unreliable primary altitude displays. (C) Boeing Aircraft Systems Manual.