Hi, this is a tricky subject and most of the info available is a bit vague, and there are lots of red herrings to be found. I've investigated one of my buys which was an easy confirmation as it was a trademark lug ( late No 3 ), correct serial number and Condor ( very helpful ) confirmed my thoughts. So my advice based on dipping my toes into this minefield goes like this : -
First confirm the year of bike - find out where was Bill Hurlow working at this time ( try the Classic Lightweights website, info can be found ).i.e. Bill Hurlow, I believe worked independantly in the sixties, doing frames for Mal Rees, H.E.Green, Condor, the Hurlow brand and maybe a few others, but he stopped making frames for Condor, etc in the late sixties, focusing on the Hurlow brand in the seventies.
What Lugs are they - are they a classic Bill Hurlow Lug, or are they maker standard. Lugless will be trickier to associate, so serial numbers may help.
Trademarks - There are certain things he did at certain times, the rear brake bridges, painting a 'H' ( some posher than others ) at the top of each seat stay, check out
http://www.classicrendezvous.com./British_isles/Hurlow/Hurlow.htm for some good examples. I checked my chainstay bridge and its lightly curved, but then so is my 1960 Elswick Hopper Lincoln Imp, which just shows the problems with this subject.
This is just a guide, and of course the main trademark is the frame, it should have quality stamped all over it. I hope this helps, happy hunting. Terry