With my guppies, (the only ones I've really noticed getting pregnant) some signs that I've noticed are: hiding out by themselves, hovering, then darting when someone gets close, males and other fish chasing more then usual, tail sort of angled up or down, and maybe more. Good luck!
Also I need advice on good sizes for anew tank and good fish to put in it I've got room for up to 100 in my basement but room for another 30 maybe 55 not sure In my room