Here's how I check 'em at work..take yer cleaning rod and push a tight patch about 3/4 the way to the barrel muzzle..You using a bore guide? well, take a piece of masking tape and wrap round the cleaning rod where rod meets bore guide, or where it meets back of the action, whatever, and wrap it so it looks like a little flag, sticking up at 12 o'clock..Pull the rod back by handle only, make sure it isn't slipping, pull until that flag is again at 12 o'clock..now take your measuring tape and measure from the front of the tape to the back of the bore guide, and that should give you a good idea of the twist rate..
I do what JR does exactly. I just do it from the muzzle end (carefully) wrapping a patch on a plastic brush so it fits tight. Push it all the way in so it's conformed and following the rifling perfectly. Then I put the "flag" of tape ( I use the colored MTM cartridge box lables) that JR mentioned flush with the muzzle and at 12 o'clock, withdraw the rod for 1 revolution exactly and measure. If you're anal about it, place another flag here too, and withdraw another revolution to compare and see how consistant the twist rate is for the length of the barrel, repeat if necessary for longer barrels.
Twist consistancy could be another good topic. [img]images/icons/smile.gif[/img]
Twist consistency, or try this one, how do you measure a 38" twist in a 20" barrel...nah, that's too easy, instead of taping at 12 o'clock, tape at 6 and pull 19" back...twist consistency you might want to use a cerrosafe cast and some math, ha..
I've never gone wrong with the cleaning rod and patch method. Works every time for me. Only problems i've ever had was with bbl's twisted at fractional rates (i.e. 1:9.5" etc..) Also, method doesn't work for gain twist bbls.