Draco's absence

[greg_a126]

No: HPFGUIDX 137358

So at the Slytherin/Gryffindor Quidditch match (American edition, page 
297):

"Oi, Harper!" yelled Harry in desperation.  "How much did Malfoy pay 
you to come on instead of him?"
He did not know what made him say it, but Harper did a double take...

So now that we've read HBP in its entirety and know Draco actually did 
pay him off, how did Harry know?  Was it just a guess?  Was it a 
particulary clever insight on Harry's part?  Or, was it some sort of 
accidental Legimency?  

Greg