Look for these signals:
"Find two elements with property X": complement lookup (two-sum pattern)
"Count occurrences" or "frequency": frequency counting
"Group by equivalence": custom keys (group anagrams)
"Subarray with sum/property": prefix sum + hash map
"Check if seen before": hash set
"Fast lookup needed": hash map When you see brute force and ask "can I do better?", hash tables are often the answer.