I used to be a freelancer, and plan to be one again someday. The thing about freelancing is risk vs reward. If you are good enough at selling, delivery, and juggling then the money you can make is astounding, especially if you can travel to wherever you need to go (client pays travel costs). This potential reward comes with a pretty high risk. There will be weeks where you have no income, if you hit a bad spot in the economy maybe even a month with no income. Having periods without income is not just a possibility, but happens to just about everyone at some point.

This means you'll need self discipline. Until you get enough squirreled to survive just on your bank account for 3mos you must squirrel away every cent you can.

It also means that 3mos is just the bare bones parachute. You need to save money for medical emergencies, car emergencies, relative visits, conferences, etc.

You also might be thinking, well I made $100k at my last job, I can make that freelancing. Problem is you are now covering your own medical insurance, and a host of other things that were cheaper or paid by the company when you collected a paycheck. As an average figure you will need 1.6 times your salary to have the equivalent standard of living. So you'd need $160k. As an aside, your company probably bills you out at a 2x - 3x multiple, so this is not as much of a cost to corporations as it sounds. Of course, when you start you may be working for smaller companies who can't hire the big firms like where you used to work, so it might be big to them.

You'll also work much harder as a freelancer if you are going to succeed. You'll need to spend between 40-60 hours a week on client work to keep clients happy (more like 30-40 once you get fully established, get a rhythm, and get regular repeat clients), then an additional 20-30 hours marketing yourself. The good news is that you're your own boss. Except...you're not. The clients now become your bosses, until you are established enough that you are turning down work, or better yet farming it out to trusted freelancer friends for a 7% cut of the work.

Marketing yourself is key. You asked how you can tell you are ready. If you're reputation is not such that you are getting at least one unsolicited query a week asking to talk to you about a job opportunity, then you're going to have to work hard on marketing, very hard. I'd recommend you find a corporate job in that case, and work on building your reputation: speak at conferences for free, contribute to open source projects, blog, participate on the professional social sites (quora, LinkedIn, stack overflow,reddit for starters). I get several unsolicited queries a week, and I don't feel my reputation is where I want it to be yet to go back into freelancing.

On risk there is always the legal risk, and you can protect yourself better...incorporate before any consulting as an S or C, and get liability insurance. Std caveat: I am not a lawyer, and no part of this post is legal advice in any way, merely an expression of personal opinion.
Have I scared you yet? If so, good. Freelancing is a roller-coaster ride full of risk, where you work harder for mostly the same std of living. If you are good, and lucky, you can make much more money than you ever would at a salary and grow your consulting into a lifestyle business. Just remember that for every 100 there convinced they are the lucky ones, one of them will make it. Don't become a freelancer as a way to get rich, the optimal approach for that is to be a hard working corporate person at a company, ideally a B-Series startup. Want more risk and potential reward? Then join a startup. For me being a freelancer is about freedom. I get to do the work I want to do (as long as money is not tight), have freedom on when to do it (as long as client deadlines are met), and do it the way I want to do it.

Some of you may have noticed...I said I was a freelancer, and plan to be one again. I got married, had a child, had several illnesses in the older generation of my family, and needed something with less risk. I'm now getting back to the point of freelancing/a software company and when I do, I will let you know how it goes.