Thanks!
A). Ask/call around your network/contacts and get information direct. This may also land you a job too.
B). Ring around the leading job agencies in the area you intend to work and ask what they can offer you based on your experience and skill set. This will give you a rough idea of what you can expect in terms of work and pay.
C). I would couple this with a look at the internet job sites and drill down to your area/market. This should give you a much more objective and clearer idea of the economy and rates too (you can gauge contract rates based on rates for full time positions). Plus the job adverts also give a good indication of the sort of skills the markets want at that time too (e.g. if you know .NET/J2EE/RoR/etc and the most money is in .NET work, then if money is your main driving factor you know you will do well to brush up on the .NET skills).
Sites like http://www.vault.com and http://www.itjobswatch.co.uk/jobs/london/microsoft.do as well as the usual job-boards are good places to start.
Hope this helps.
http://freelanceswitch.com/rates/
If you're charging too much, you'll find out soon. If you're charging too little, start increasing your rate.
Ask YC: What are the going rates for consulting gigs? http://news.ycombinator.com/item?id=182369