Petunia why care? (spoilers)

[Gwared]

If Dudley was in anyway magical he would have recieved a letter from Hogwarts.

[June]

If Dudley was in anyway magical he would have recieved a letter from Hogwarts.

Well... maybe not. Those students accepted into Hogwarts would probably have enough magical power to get them in in the first place. Remember that Neville's family didn't think that he had enough magic in him to be accepted into Hogwarts although they knew he was not a squib, so it was a pleasant surprise for them when they found that he was. I should think that only magical children with a high enough level of magic are accepted into Hogwarts... the others with lower magical ability would probably learn from other wizards... like an apprentice, maybe? So, it's still possible that Dudley have magical powers, just not enough to warrant an entry into Hogwarts...