Back when I was racing, and before I was measuring, I had a list of things that indicated a course was short(even before I measured I knew that an inaccurate course almost always meant a short course). Others can feel free to add their own.
1) The start and finish were in convenient locations and there was no turn-around.
2) The mile splits were wrong, unless the wrongness cancelled itself out in consecutive miles.
3) There was no 3-mile split or marker in a 5k or no 6-mile split or marker in a 10k.
4) The race didn't start on time.
I also had a less-than-serious list.
1) The race director was <name deleted>.
2) Your last mile was your fastest.
3) You set a PR.
4) Most of your friends set PRs.
5) Most of your friends over 40 set PRs.
6) You ask the race director if the course is accurate and his reply is:
a) How do you mean?
b) Yes, I measured it myself.
c) Did you get a bagel?
7) During your warmup you see somebody driving the course holding a measuring wheel out the driver-side window. (actually happened)