I've played games with some friends and they claimed that so-and-so was cheating. I honestly just thought that he was a really good player.
What are some indications that someone is actually cheating?
It is very hard to tell nowadays, where you can rack up kills on tight maps like NukeTown by using Hardline and getting a mass destruction weapon like the Gunship or the Chopper Gunner.
Speed hacks (he moves much faster than anyone ever can) and health cheats (he just does not die) are more obvious, so I am not going to mention them
Wall hacking is the most commonly used type of cheat where one can see through walls and track enemies from very far distances.
Some indications are:
There are some caveats to the last one, though. He might know you're there because:
There is one test to almost exactly tell if he can see through walls: the step-ahead-step-back technique, in which you need to know the place the player in question is before hand. As you are coming out of a closed space or round a corner, just run to the corner and before you really show yourself, back up a step. If he fired before you appeared chances are high he was tracking you through the walls.
As you can see, finding out whether someone is cheating is not an easy task. He might just be a good player, having a strong intuition or excellent knowledge of the map, and a good timing. I would really not take this as a rule set to say yes or no, but as a checklist of questions you ask yourself before accusing someone.
For sake of completeness, check out Toastandjam18's interesting answer below about boosting.
Some clear indicators are:
What must not be confused as cheating:
Could you use the theater mode to replay the game from their perspective? This may make cheating more apparent and would provide you with evidence. The only caveat would be I'm not sure how accurately theater mode reproduces the game given players experiencing various degrees of lag from the server/host.
There is also boosting in which the player uses a switch between his modem and PC/xpox/PS that cuts the connection long enough for him to jump places on map but not disconnect from the lobby. This is easily seen on kill cam or in game. Note that some people have a bad connection like when i played in iraq but you can check that in the menu, if he is full strength then red barred then full strength again, then has a high score chances are he/she is boosting
Another common thing I've seen is FALs shot in an extremely fast and predictive manner. This means they are cheating by having a special controller that sends the fire button in repetition. This is usually seen very easily in kill cams where even when starting and stopping firing, it is still being shot at a constant rate(rather than like most people who gradually speed up and slow down)
One I have noticed, though it may just be a bug in the game itself, is when you get them in your sights and your gun automatically "shifts" away from that player. I thought I was going nuts, but one day I had put my controller down for a sec to grab the phone, and some geezer ran through my crosshairs and pushed it without me touching the controller. This theory was proven fact when I followed him, tried to get a bead on him and I couldn't.
I have played enough Black Ops to know. These conditions indicate cheating:
I have also heard a few dumb players brag about their cheating while playing the game. There are players who are very very good who do not cheat.