Hi LS,
Thanks for the message! This is a topic that comes up recurrently in our books and courses, so you'll see it addressed repeatedly
The first question I always ask myself is: does one of these sets have an obvious order? That could order of any type, including numerical/ranking/order (1-2-3-4-5 etc), grading (A-B-C-D etc), days of the week (Mon-Tue-Wed etc), or anything else they come up with. If I see one of those sets, that's usually going to be the base, and we almost certainly have a linear game in play. Watch out for sets that look like they have order but don't (see my next question for an example of that).
The second question is: if there's no order, does one of the sets naturally "hold" or contain one or more of the others sets? For example, imagine a game with 9 adults and 3 canoes. The canoes naturally "hold" the adults, so intuitively they would work as a great base. Incidentally, let's say those canoes are numbered 1, 2, and 3. The initial thought might be that 1-2-3 has order and that makes it the base, but chances are pretty good that the ordering element doesn't matter and it's a Grouping game not a Linear game. Regardless, the canoes end up as the base due to their grouping power.
Those two questions handle the majority of base selections you would make, but the best way to get good at this is to study game setups and see what choices we made for each base. you'll begin to realize the above questions handle mos to it, but there are unusual exceptions--study those especially closely!
Hopefully that gives you a good start. Please let me know if that helps. Thanks!
.