How do I know if my Mobo will fit?

I've picked out two possible cases,

Choice #1
Choice #2

(in no particular order)

Now, looking at the inside views of both cases shows the places where I would screw my mobo in. The Xclio case has a LOT of holes for screws, and I can't imagine my mobo not fitting it.. (I'm not sure what mobo I have, all it says in PC Wizard is "Pegatron Corporation Viola"), but when I look at the Antec case, I see about 5 holes. How can I find out either what mobo I have, or if it'll fit?

Compatibility is a form factor thing. Both those cases are ATX, they will support both their namesake and mATX motherboard's. Most modern mobo's are still relegated by this specific form factor, so you won't likely run into any problems with compatibility.

Use Pcwizard to find the make and model (if you dont know it) of your board if you need to be certain.