You can’t be sure who a twitterer is from their username, their profile, their link, or their location.
In fact you can’t be sure at all and so should treat all twitterers as suspect sources.
The only way is to contact the person directly, or visit their own website and see if they link back to the twitter account.
Twitter is working on this: they have introduced a Verified Account feature which in theory will mean that those who want to, and who are able to, can verify they are who they say they are.
The verified account badge will appear on the home page of the user:
but not on the photo/avatar/icon of the user (easy to fake.)