Signal 1: DP transition with over a sliding window . Signal 2: "Find shortest/longest subarray with some property" where property involves sums or min/max.
Signal 3: Two-pointer problems where you need efficient access to window's extreme values. Not applicable: arbitrary range (use segment tree), range sum (use prefix sums), or when window bounds are non-monotonic. The sliding window with min/max is the telltale sign. If you see a bounded range of indices, think monotonic queue.