Well, you'd need a thread pitch gauge and something to measure the bolt diameter. A vernier or digital calliper would do for the later:
http://www.maplin.co.uk/Module.aspx?ModuleNo=32198
and you can get thread gauges here:
http://www.axminster.co.uk/product.asp?pf_id=23483
You may find them cheaper on eBay - my digital calliper only cost me a 50p (but I did have to wait for it to come from Hong Kong).