The easy, but not extremely accurate, method is to drop a bullet into the chamber pointy end first. This is most easily done by sliding the bullet down a cleaning rod guide, then giving it a LIGHT tap with a cleaning rod to make sure it's seated in the rifling. Then carefully insert a cleaning rod or small dowel into the muzzle until it contacts the bullet. Mark the rod (with no jag) or dowel with a sharp pencil at the muzzle, then tap the bullet out of the rifling and remove it from the chamber/action. Install the rifle's bolt, and insert the cleaning rod or dowel into the bore and push it carefully all the way until it contacts the bolt face. Mark the rod or dowel. Measure the distance between the two marks, and that will give you the maximum OAL with the bullet you used. It's not super accurate, but it's better than nothing.
If they used a reamer that cuts a very long throat, for example one that gives a single loading chamber you may not be able to get close to the rifling unless you're using 80 gr or heavier bullets. That would be sort of silly unless the barrel has a 1 in 8 or 1 in 7 twist, but manufacturers sometimes do things I think are pretty silly.
