A small mirror tile can be a useful tool to add to your toolkit. It can be particularly helpful when looking for kinks, dips and misalignments in track.
To check alignments simply set the mirror on the edge on the track. That will enable you to inspect the alignment of the rails, not by looking directly along the rails, but instead by looking into the mirror at the image of the rails.
This will save you from stretching or bending over (no so easy as we get older) or getting track imprints in your cheek. When you bend down and sight along a rail the track can appear to be very smooth. However, by looking at the image of the rail in a mirror, kinks and dips are often easier to spot.
The mirror can also prove useful when placed alongside the track to view the action of trucks and wheels while the train moves over a trouble spot.