Complex sentence: If there is no BOM or other indication of the encoding, heuristic analysis is often able to reliably determine whether UTF-8 is in use due to the vast majority of byte sequences that are invalid in UTF-8.
Split into simpler sentences: If there is no BOM, heuristic analysis is required to determine if UTF-8 is by a file or not. Determining if a file is UTF-8 is quite reliable due to the vast majority of byte sequences that are invalid in UTF-8.