Easiest thing is to see where the email has been sent from. If you hover your arrow over the title of the email, the sender's address should be shown.
If it doesn't look right, ignore it!
Things like being incorrectly addressed to you at the beginning of an email is also a bit of a give away - if they can't be bothered to get that correct .......
I will say, having worked in a hospital for a while, all letters and emails have to be addressed using the correct form , by name, not "Dear Sir" or even "Dear patient"!
Just check on the website of where it has supposedly come from - GP surgery/Medical Centre or hospital, just to set your mind at rest - both will state if there have any changes either in their news or updates