Home Decor Projects Observer

Hello. So I'm about to tackle a tiling job I've been putting off for a while.

I'll be needing to remove some old red oak flooring, but I do not want to cut into the sub floor. How can I determine the thickness of the oak planks I currently have without doing that?

I have no vents or registers to look at. I don't mind actually cutting into a corner or something, possibly hitting the sub floor if I need to do that to find the thickness of the planks.

Thanks.