I know in my experience with Verizon, if I start a call at 8:55pm and talk until 9:45pm, the first 6 minutes (8:55 to 9:00) are peak and the remainder (9:01pm to 9:45) is off-peak. I've seen it clearly on my bill. I'm on the 1500 Anytime minutes plan with T-Mobile, so I can't comment on their Peak/Off-Peak practice since it doesn't apply. I don't use my AT&T number at night so I haven't experienced their method of Peak/Off-Peak tabulating.
For every carrier night and weekend minutes are very clearly stated. I think the two things most consumers know about their plan is when their nights start and how many peak minutes they have.