Begin with the room
Stand where the plant will live and watch the room on two ordinary days. Note when the glass admits direct sun, and where nearby walls or trees cast shade.
Consider distance
A bright-looking room is not equally bright everywhere. A shelf two metres from the window can receive much less usable light than a sill just beside it.
Make one adjustment
Move a struggling plant a modest distance rather than changing its soil, water and location together. Compare the next few leaves with the older growth.
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.