How can one verify the truth behind historical documents? In other words, how do historians ensure that what is written in documents is not falsely glorified?

Answer 1

There is no way to be 100% certain with anything, but taking a holistic approach to answering doubts in accounts is usually beneficial to providing a more rounded truth.

If you have other historical accounts, then that is extremely helpful! Especially by the opposing party. Verifying a historical account with archaeological studies is something that is also tremendously helpful in weeding out inconsistencies. Working with geologists, psychologists, engineers, astrologists, etc... to study a record and a site is a great way to put some perspective on the account.

Keep in mind that nothing is infallible though. It's possible that something was noted in the account that is true, but archaeologists weren't able to find that particular thing or they interpreted it incorrectly.