Dolby Digital EX is usually identified by your receiver if the EX flag is enabled on the DVD. If you have a 6.1 or 7.1 setup, you'll realize that many DVDs from 2000/2001 upwards are EX encoded. And even if they're not, your receiver will still treat them like EX DVDs by automatically matricing the sound across your rear channel(s). Your receiver display will give you the right information. Doesn't matter if it's an R1 or R3 release since the studios don't usually re-encode English language Dolby audio across regions.
I'd like to emphasize that Dolby EX for HT isn't true 6.1 unlike DTS-ES. It simply extends the 5.1 surround field to use additional speakers located behind you creating a matrixed 6.1 or 7.1 listening experience.
More details in my post
here