Watch the clock
Early sun may be gentle while a late afternoon beam is intense. Check which hours the plant really receives sun instead of estimating from the room as a whole.
Read slow changes
A stretched stem or longer gaps between leaves can indicate insufficient light. One pale leaf, on its own, is rarely enough evidence to diagnose the cause.
Mind the seasons
The angle of sunlight shifts during the year. Recheck familiar places when the seasons change, especially where leaves may touch hot or cold window glass.
Try a paper shadow test
Hold a sheet of paper beside the plant at midday. A distinct shadow suggests direct light; a soft one suggests diffused light. Repeat on another day before drawing conclusions.