How do I check if my smartphone will work in Thai Phone Networks?

[topshot]

I'm looking to buy an unlocked new/used smart phone back in the USA and wondering how to check if it will work in Thailand.

I am looking at an unlocked Samsung Galaxy S2 or a S2 Skyrocket. (currently looking at AT&T's new no contract monthly plan for $55 and only allows S2 and not S3) But if I can find another carrier, I would consider getting a used Galaxy S3. But I am not sure if it will work in Thailand. I am a complete smartphone newb.

How do I check if the smartphone will work when I switch phone companies in Thailand?

I know many people from the states bring their iPhones, but even there are differences between AT&T, Verizon and T Mobile unlocked models.

And I am wondering how much prepaid wireless/data would cost in Thailand. I mostly want the smart phone for Skype/Google Voice for when I receive a call from the states or need to make a call for business.

[Kudel]

I used to have an S2 mini it was a quad band worked fine on Thai networks last year.

[Hedghog]

If the device is unlocked.

Switch to a local sim card on arrival no problem.

[Kudel]

Or if you dont like switching sim cards have a look at smart phones with dual sim wich means you can have a sim card from youre home country and a thai sim card and with most models its very easy to switch from one sim card to another.