Mostly for the US mantids (Chinese, Carolina, European)
1) they will refuse food. (If you really pay attention to your mantids, you will be able to know the difference between a "hunting" strike and a "leave me alone" strike.
2) while hanging upside down they do more of a longer hang like stretching their legs.
3) know that they usually molt every week-two weeks if you're feeding pretty consistently. (Keep track of previous molts thatbwill help you)
Rick said it in a post before, there's gonna come a time when you "just know" when your mantis is going to molt but until then those are the pretty big signs. There are many more but I think those are the big ones.