The interim market is dominated by a relatively small number of agencies (some are active in many markets, others are specialist to particular trade sectors/professions), so that's your first port of call, to look at the recruitment marketing etc and find which agencies are active in your field advertising for jobs for which you meet the requirements, and then contact them to discuss your qualifications and experience to gauge demand and remuneration.
Then you need the courage of your convictions to start applying for jobs and see how many interviews you get, how many offers, what remuneration packages are on offer etc. If things are quiet, then may not be such a good idea to give up a steady job, as if it takes a while to get your first contract, it'll probably be slow to get the next too, so you could have quite long "resting" times between contracts. If the phone is too hot to handle and you get loads of interviews/offers for suitable jobs, then that proves you're skills are in demand and you'd probably make a better career out of it.
Just be prepared to be messed around, to work long hours, to risk early termination etc. It's different to a job. Clients may mess you around with starting dates, contract durations, etc. You may be required to work longer hours than the other workers, maybe more travel involved to other sites for meetings etc, maybe working the midnight hours at certain points in the contract to meet deadlines. Also, be prepared for the downsides such as not getting paid when you're off sick and not getting paid holidays (if holidays are allowed at all during the contract). Risks of early terminations are a lot higher, i.e. easier to terminate a contractor for breach of contract than it is to sack an employee for poor performance, risk of the client (or agency) going bankrupt and you not getting paid, etc.