use burp suite and try all the lfi based payloads on the parameter

Best way to find LFI, use intuder and match this:

root:[^:]:[^:]:[^:]*:

https://github.com/rix4uni/WordList/blob/main/payloads/lfi/lfi-medium.txt

when there is a parameter try fetching the local files .

use absolute path first and then use canonical path

irrespective of the functionality of the parameter whether it is fetching a page or fetching something we have to test for LFI there