Yep, on the fat/widest end of the egg. Some eggs are oddly shaped and it’s hard to tell which end is which, but usually if you candle them you can find it. If you can see them, mark them.

Sometimes fresh eggs aren’t super obvious. In that case, just mark them the next time you candle. Always candle before setting them to check for cracks, too, since these are not usually good candidates for hatching. (Sometimes you can try to save them by sealing the crack but it depends on the crack.)
Also, because I have to add yet more, lol, sometimes in shipped eggs you’ll have wobbly or detached air cells. I usually mark these eggs so I know they have a not-so-perfect air cell, but don’t do anything different with them. I like to quantify
everything.
haha!