Look for these signals:
Contiguous sequence: The problem asks about subarrays or substrings, not subsequences.
Window optimization: Find longest, shortest, or optimal value over a contiguous range.
Monotonic validity: Adding elements only makes the window valid or invalid in one direction. Removing from the other end can restore validity.
Original order: You process elements as they appear. No sorting or rearranging.
If shrinking the window can't restore a broken constraint, sliding window won't work. Consider DP instead.