Wow, that's a puzzler. How indeed do you know you're ready? In absolute terms, you don't. You just make the best guess you can based on the information currently available to you.
If you think your marriage (or other "original relationship") is on good footing and then open the relationship ... only then to find out that it has cracks and weaknesses ... Hmmm, I actually think that commonly happens. Polyamory is infamous for applying a spotlight to cracks and weaknesses not previously known in the original relationship.
So first of all, I guess my advice would be don't panic. Just apply the brakes a little and give yourself more time and breathing room. Start working on the weaknesses that you've newly discovered in the original relationship. In the meantime, I tend to think it's alright to keep the relationship open -- just add caution to the way you keep it open, and you may need to negotiate some new (perhaps/probably temporary) rules to the "agreement to open" just to be safe.
But no I don't think you can *know* you're ready. There's a certain amount of intuition (and limited experience) involved in making these judgment calls. Usually not the end of the world if you find out you jumped the gun, although it does depend on the specifics of your situation.
Hope that helps.
Love means never having to say, "Put down that meat cleaver!"