The question of “being ready for marriage” can’t be detached from the question of the partner. You can be ready for marriage only with the right partner. Otherwise, the question is void of meaning.
Also, I don’t think it’s helpful to approach a question of marriage with the mindset “I’m looking for a person to be with to the rest of my life”. This sets you up for unreasonable expectations.
So I’m transforming the question, into “having a serious partner, how do I know if I should marry him/her?”
Assuming you have feelings for a person, and enjoy being with him, and generally ready for serious relationship, than then only question you must be able to answer is “do you see a future together with this person?”. While this is somewhat vague question, answering it is easy, if your partner is “marriage-quality” for you.
Seeing a future together means that you can imagine your future life, in it’s various transitions and changes with your partner “fitting” the picture you imagine.
Traveling together (you want to travel, is she with you?), settling down in a small city (you want to live in a small quite suburb, is she with you?), having kids (you want to, do you see your partner as the mother?), changing career (you are pretty sure you will want to do something different in 10 years, do you see her supporting you in this painful transition?)
If you are able to see your partner in (your own) imaginary scenes of your future live, than I congratulate you – you found your future wife/husband.