The best tool I have found for finding the lands is the R-P Products C.O.L. E-Z Check tool shown here with the Hornady Gauge
It is essentially a steel rod with 2 collets with set screws. You just insert the rod in the muzzle to the bolt face and lock the rear collet
then insert the bullet into the lands and hold there (the Hornady tool makes a good tool for this), and insert the rod to hit the bullet tip and lock the front collet
and measure between the collets.
The Hornady tool will give you false readings since you insert it to where you have contact at the shoulder. There will be a difference because the bolt will hold an actual case further back. A solution to this is to drill and tap one of your own fired cases that has developed a crush fit.
The R-P tool contact info is
[email protected] or (318)424-7867 the last time I checked. I got mine several years ago and I think the price is still $25.00, but I haven't checked lately. Essentially it is a commercial product that accomplishes the same thing as marking a dowel or cleaning rod, except with much more precision.
BTW, seating a bullet in a tight necked case to the lands may work but the lands have a habit of grabbing the bullet and pulling on it. Just something to watch for.