Depends on the child. Mine didn't show huge signs beforehand.
Dd1 at 22 months was told by an older friend "you're a baby you wear nappies". She was dry during the day from the next day. Had shown no signs of even wanting to beforehand. She sat on the potty that evening and went, and from the next day was dry with hardly an accident. She had one accident when 3yo after the first week of training and no others since then.
DD2 had lots of signs from about 18 months. About 20 months she wanted pants like her big sister, would sit on the potty, tell us she needed to go, and was dry consistently overnight from before 18 months. She was dry before she was 2yo generally but had the problem she'd leave it to the last minute to tell you, and in a town where all the public toilets are down one end wasn't the best, so she used to wear equivalent of pull ups when out.
Ds showed no signs, but I realised he was going to have to train during the summer holidays as I had a round trip of an hour and a half (walking) for picking his big sisters up from (two different) schools. So I thought I'd give it a go in the summer holidays he was 25 months. First day wasn't great, then second day it clicked, and he also refused a nappy overnight. Again he hardly had an accident day or night after that day. Really the easiest boy training I've come across.
So dd2 showed the most signs and was the hardest.
We did use washable nappies though, which do make a difference.