How Do You Know If a Poem is Good or Bad?
The question is misleading, and opens the door to trouble. Lots of self-appointed critics will try to convince you that their criteria for selection is the one true religion. There are objective criteria, and there are also subjective ones. Beware the authority who insists that their subjective judgment is an objective criteria: they will never give you the whole map, just their tiny corner of it. Always remember that it is much easier to tell if a poem is well-crafted than if it is great.
The truth is: It's an impossible question, with some conditional answers.
But I can tell you how to find all this out for yourself. Here's how:
Read, read, read, read, read, read, read some more, then read. And keep reading.
The more poetry you read, good or bad, the more likely you are to be able to spot something that stands out as great. You are also more likely to learn about clichés and how to avoid them. The more you read, the more you can see what works, and what doesn't. There is no substitute or short-cut for actually reading a lot of poetry. Apprenticeship takes time.
Anyone who claims that all you need to read, in order to write great poems, is their little style manual is lying. They are full of themselves, and wrong. There is no magic bullet.
Each poem is a complete universe, and must be judged on its own terms. Approach it as if you are stepping through a gateway into another world. Dive in, and wade around.
Leave the critical theories at home. If you must read them, read them after you've read the poems. Learning the craft or writing is good, because you need both inspiration and craft, working in harmonious balance, in order to create an outstanding poem. And you can learn a lot about craft just by reading a lot of poems. But do not think that you have arrived at wisdom if all you have studied is craft.
If you want to learn all the linguistic, grammatical and syntactical technical terms in writing, go ahead. But never confuse the trees for the forest. Those tools will help you analyse and understand a poem, and maybe even help you revise one, but they won't help you create one. Never let the tools of analysis dictate what you write; as wonderful as they are, that is not their proper place. If you manage to study the "rules" enough to have internalized them to the point where you don't consciously think about them when writing, all the better. "Learn the rules, forget the rules, break the rules." Which really means, "Transcend the rules."
Remember that academic "creative writing" programs, such as all those shiny new MFAs in poetry, can only teach you analysis and criticism: they are incapable of granting wisdom, or teaching inspiration or vision. They can teach you the tools of analysis, but that's about all they teach. They can help you hone your vision and your voice, if you already have one; but they cannot provide you with one.
I keep returning to what poet Adrienne Rich once said about what poems should do: Instead of being about an experience, a poem should be an experience.
Note that Rich did not say "a poem should move you," or that it's all about feelings. Feelings are only one kind of experience. What she implies is that a poem should pull you all the way into its world for the duration of reading, so that you forget all other words. A poem should be immersive—not only either intellectual or only emotional. It should be a complete experience, in and of itself.
And don't forget to keep reading more poems. If that's all you ever do, that will suffice.