Q: Can one window in a group be tested for lead?

No.  The certified renovator must test each component affected by the renovation.  If the components make up an integrated whole, such as the individual stair treads and risers of a...

No.  The certified renovator must test each component affected by the renovation.  If the components make up an integrated whole, such as the individual stair treads and risers of a single staircase, the renovator is required to test only one of the individual components, unless the individual components appear to have been repainted or refinished separately.  Multiple window sills are not integrated parts of a whole.  They are separate components and must be tested separately.