It's called differential diagnosis and clinical know-how. If a child has lower deciduous second molars which are just getting loose, that is the prime indication to start treatment immediately. Loss of the lower deciduous second molars generally occurs at the same time as the growth spurt starts, but may be delayed by other factors. The key here is to start treatment before that growth spurt occurs. You put them on progressively shorter recall.
Clinical judgment is invaluable and it's the doctor who makes the difference.
When I start full treatment, I only place bonded brackets on the upper teeth from first molar to first molar. Because the upper teeth determine the ability to accomplish full alignment of both arches. If you can't satisfactorily align the upper teeth into a balanced relationship with the face, then you have to make a decision to extract. Blocked out upper cuspids are not a problem which absolutely dictates extractions. Sometimes that decision can be made upon clinical inspection and evaluation of the face, but other times, a trial non-extraction decision is made.