We do know that Lily gave Petunia a very strong potion to change her appearance, and it worked. It seems reasonable for Harry to assume that that potion would only work on Squibs, not Muggles.
Also, from chapter 7: ” There were no questions about his father accompanying him to the magical side of King’s Cross Station. Dad had trouble just looking at Harry’s trunk directly. Magic ran in families, and Michael Verres-Evans couldn’t even walk.” The fact that Michael is singled out here suggests that Petunia doesn’t have the same problems interacting with magic.
I remembered the bit about the chest, and I accepted that as showing that Michael’s a Muggle, but it’s vaguer about Petunia. But I forgot about the potion that had already worked on Petunia; that seems pretty clear. So thanks!