I've been thinking more about your question.
I think that there are lots of things that you can work on and ways of figuring out how or when you might be ready for a poly (or mono) relationship. I should say that I am very much of the opinion that there are lots of people in relationships where they are causing massive amounts of damage - because they got into those relationships before they were ready and without thinking about it. This is not a problem that is unique to poly - it happens in mono relationships. It happens with children, with pets etc.
So thinking about it and figuring out ways of dealing with the things that you can predict might come up. Reading these boards gives a good indication - people deal with and write about all sorts of issues so there are tons of examples just here. Have a plan. You might need to change it but at least have one.
Be honest about the things that you might find difficult and devise ways of figuring out what they might be.
Jealousy and coping mechanisms for it is an obvious one. The signs are there in most lives.
If you have a dog - how do you feel when they get a hug from somebody who isn't you? Are you pleased that they have a new friend or do you take them away and tell the person that you don't like them hanging out with strangers. Does your dog not even hang out with new people because your preference is that you are the special person in their life? (if the last is you, you probably need to do a ton more work before starting on poly)
How do you cope if your dog has an interest that you don't like or don't understand? Do you prevent it? Do you work on your own comfort zone until you can find ways to help and encourage the dog in their interest? Do you not know what your dog's interests are (if not, what makes you think that you'd be able to notice, cope with or engage with a partner who has different interests)?
If you have a romantic partner who you are currently mono with, how do you feel if they come and tell you that they've made an interesting, attractive and wonderful new friend? How do you deal with your feelings? Are you able to be excited for your partner or do you find yourself nagging them and putting barriers in the way of the new friendship? I'd see nagging and barriers type behaviour as signs that poly might not be a good idea right now.
Do you and your romantic partner not ever meet new people or make new friends so you don't know about how you might react? If that was the case for me, I'd want to work on being able to make new friends and form new connections without the added intensity of them being romantic connections. I think it's important to not run before you can walk.
If you have a romantic partner, how do things go when things get tough or something interesting happens? Does one of you regularly feel abandoned by the other whenever something new comes up? Or are able to happily maintain your connection with each other even when a new thing has happened? If feelings of being abandoned happen or lack of time together is already a problem, I'd probably push poly back a bit.
If you have no romantic partner, how do you find things go when your friends make new friends/get a new partner? Are you happy for them, worried about lack of time with them, annoyed that your place in their life might be under threat. How do you deal with your feelings? I have friends who have encountered a range of reactions from physical violence being directed at them if they tried to have a new friend, to nagging and barriers, to friendships ending, to friends being happy and new connections being forged between everybody.
That sort of thing will give you a good indication of how you cope with changes in your social life and support structure. If you tend to punch your best friend if she gets a boyfriend, you probably aren't ready for poly.
If you are single and have no friends - learning the skills to make friends and keep connections going is where I would start rather than romantic relationships of any description.
Lots and lots of ways to figure out how you might cope and situations that allow for learning of skills that would make for easier, more successful poly relationships.