Everyone is different, but the following are some approaches I have used when self-studying.
Look at the exercises. If any look particularily scary or you have no idea how to begin on some, then do that exercise.
Another approach is simply to do the last 5 exercises or so in each section (unless they are very similar, then just pick 5 late exercises that look different). They tend to be the hardest so if you have problems with those try some of the earlier ones that looks to have a similar theme, and once you understand the ideas go back to the later ones.
The idea is that ideally you should be able to solve all exercises, but only actually solve a few. It is pretty hard to pick exercises because books like Dummit Foote are optimized for classroom exposition, and therefore way too many exercises are included so professors can pick and choose.
If you tend to breeze through all exercises, then you may be doing too many. If you tend to struggle with most exercises you are likely doing too few, or too easy ones.