]And how does K. guess that Emma is trying to match her friend with Elton?

Mr Elton seems to be the only unmarried man who Emma spends time with who could be a possible match for Harriet. There are three unmarried men in Emma's life (AFAICR):

Mr Woodhouse - it would be unrealistic to imagine Emma planning for a match between him and Harriet

Mr Knightley himself - not realistic either, IMO, and IF Emma had wanted to match him with Harriet she would have made some moves towards that which Mr Knightley undoubtedly would have noticed

That leaves Mr Elton, "a young man living alone without liking it, the privilege of exchanging any vacant evening of his own blank solitude for the elegancies and society of Mr. Woodhouse's drawing-room and the smiles of his lovely daughter, was in no danger of being thrown away." (Ch 3)