It triggers emotions
This is a point that can’t really be measured, but determines if this poem is right for you personally. If reading the poems makes you feel a certain way or triggers strong imagery, this is a sign that the writer reaches you with his words. If that’s not the case, maybe consider searching on!
You understand the poem
This might be an obvious point, but still is very crucial. Poetry is sometimes very subtle or very metaphorical and it sometimes takes some thinking to determine what the author is actually trying to say. Especially some of the classics are not that easy to understand, which does in no way make them bad writing, but maybe not suitable for your personal use.
It has a heartbeat
Poetry should have a certain rhythm to it. Try reading it out loud to determine if it has a “melody” and doesn’t feel like you’re reading something a thesaurus cooked up.
It is aimed at someone specific
A great poem at least feels like it’s addressed at someone specific, someone special. Read it and check if it feels personal.
You just love it
Even if all the other points don’t work out. If you just love this poem and can’t even explain why, it is still a great work of art. If it moves, touches and inspires… it’s just wonderful!