The Constitution ability (PHB 177) lists tasks like the following, in which the DM might ask for Constitution checks:
Hold your breath
March or labor for hours without rest
Go without sleep
Survive without food or water
Quaff and entire stein of ale in one go
But the 1st, 2nd, and 4th instances actually have specific rules that ask for saving throws, not ability checks. For example, the second bullet states "March or labor for hours without rest," but there are rules for Forced March (PHB 181). Those rules state that you can march for 8 hours at a slow/normal/fast pace, and then must make a DC 11 Constitution save on the 9th hour of marching, DC 12 on the 10th hour of marching, etc., or take 1 level of exhaustion.
Meanwhile, the 4th bullet point, surviving without food or water, is detailed in PHB 185. A character can go without food for a number of days equal to 3 + their Con modifier and automatically take one level of exhaustion (no save) per day beyond that, and they can go with only half their water requirements if they make a DC 15 Con save per day, or otherwise take 1 level of exhaustion.
So, if I wanted to ask a character who is going to labor for hours without rest to make some kind of roll, should I ask for a Constitution check or a Constitution save? How should the two be separated/chosen from?