bite someone on the arse

[Lindt]

today i watch a tv show named The fades,and there is one sentence that i can't get it.
"Armstrong wouldn't know a suspect if one bit him on the arse."A is a policeman,and there were so many people missing which were killed by fades,as a human,the policeman didn't expect these crimes were made by fedes,so i think it probably means he couldn't find the suspect. ps,when i google this,i saw many results turned out to be with"woudn't/don't know something/someone if it bite him in the arse
so anyone can help me with this?waiting for your answer~

[Route21]

It means:

"Armstrong wouldn't recognise a suspect even if their guilt were abundantly obvious to everyone else".

Hope this helps
R21