what evidence would show that a used remington 700 has been trued?

[Dennis Sorensen]

Chambering a .308 to 30-06 will most likely require setting the barrel back considerably and rethreading and chambering. The 308 chamber is fatter at the shoulder than the 30-06 is at the corresponding length. Depending on the present chamber and the 06 reamer, it can vary a lot.