How is “to be sure” used?

In Tenet (2020), one line is: “That's the only way to be sure.”

Which movie is “to be sure” from?

The clip at the top of this page is from Tenet (2020).

to be certain, or to make sure, often as a check or precaution; used to admit that sth is true

formal · Heard in 25 movies

Editorial90% of films
to be certain, or to make sure, often as a check or precaution
Oxford10% of films
used to admit that sth is true

Definitions from the Oxford Advanced Learner's Dictionary, 9th edition

That's the only way to be sure.

Said in 8 films · #1