When the temp starts rising again and the smell changes.

Collect small amounts at a time. About a pint and 1/2 in a quart jar is my usual though I will stick a larger jug under it for an hour or so once it is stable in the middle and I need or want to get away from it for a while. Refluxing a stripped run I will usually collect all of my heads in the first jar and tails in the last one. The cleanest smelling and tasting jars are the ones I keep and carbon filter in any case. The others get tossed into the next reflux run. I usually collect from 78 to 93 or 94 degrees. I switch over to pot stilling once the tails start coming over strong. It is easy to do with the valves on my cooling lines. Temperature though will depend on your thermometer and exactly where in the column it is placed. Even moving it 1/2 inch up or down in my 2' column makes a difference. I have adjusted the placement of mine to agree with proper temperature as assured by corresponding smell and taste results. You really do have to rely on all three of those indicators instead of just one. In order IMO they come smell first, temp with good thermometer placement second, and lastly, taste should agree with the first two. Nothing replaces a little experience though. Just do it and learn it.