Ideally if possible try a mask first in a shop before you purchase. To check the fit the easiest thing to do is to put the mask on your face without using the strap and suck just a little, the mask should stick because of the vacuum formed. Hold it with no hands just the vacuum for a few seconds, checking to see if there are any leaks, if the mask falls off then it fits badly.
Put the strap on and check it feels comfortable and does not irritate. Your mask should not be overly tight, in fact just a little pressure almost to the point that it feels loose, that would be the most comfortable.
You can tell divers with overly tight masks as they have a ring around the face after the dive.