(The recent DWM with Jenna-Louise Coleman on the cover has an article on DW actors on Twitter and includes a caution about the fakers. It's one reason I haven't bothered going on Twitter myself - I mean, do we know for 100% certain that those Twitter posts attributed to Moffat, Sophia Myles, Karen Gillan, etc are REALLY from them?)
For celebrities, Twitter does to a verification. Generally it'll have like a green or blue check mark next to the name, meaning it's been a verified profile. So, yes, 100% certain.