My understanding of an ILS is that the only significant (powerful) false gideslopes occur above the normal beam at multiples of the glideslope angle (theta) e.g. 2 x theta, 3 x theta, which are 6 and 9 deg for a standard 3 deg slope. Also that the sense of the beam reverses at each multiple, thus only a ‘9 deg slope’ can be flown in the normal sense. Thus the hazard of a false glideslope is often overrated; the more significant hazard is not checking or incorrectly checking the altitude and range. LEM covered this point previously.
Also of significance are the approaches where markers have been replaced by DME, and where the procedure chart should provide an altitude / range table. How or when are these tables used, who has a SOP, how many times do you check altitude and range?
A good safety fall back for NPAs is to use the Rad Alt. NPAs are designed so that there will be terrain (obstacle) clearances of 1000ft, 500ft, and 250ft RA when outside the IAF, FAF, and MDA respectively; thus if any of these altitudes are seen within the range / fix positions then a go around should be flown.
Also, (and without RA) there are the very useful wind/GS based timing and VS calculations that aid situation awareness and provide a cross check, now how many of us use these?