I was going to give a class on checking your own mezuzah, but it got prevented by weather. Today I found the notes I’d made for it, so I’m typing them up.
But I’m not putting in the pictures here because that’ll take hours of scanning and photographing; if you want those you’ll have to get me to come give the class.
Anyway, this is what we would have covered:
Is the mezuzah written on parchment, or printed on paper?
Does it have all the words? (checking using a tikkun; checking using the internet)
Are all the original letters there? (flaking, damage)
Are they still in good repair? (cracking)
Theoretical interlude: why repairing letters out of order is problematic
Technical bit: checking to see if the letters have their proper form (this is the bit that requires lots of training and practice, but there are some things that you can see straight away, like letters which are connected, or really egregious malformations).
Such a class is not going to equip the average mezuzah-user to say “Yes, this mezuzah is definitely kosher,” but it will equip said user to know if their mezuzah is not kosher, or if they should be worried.
As ever, for more information I heartily recommend the book Tefillin and Mezuzos: A Pictorial Guide by Yerachmiel Askotzky. You can buy it here at his site.