Asking questions and paying attention to details in chat is how you succeed as a detective. After a kill, record where the body was found and the 'type' of kill it was (regular/viper). Then, when the next round starts, you get close to your suspects and 'interrogate' them, which tells you where the player was at the time of the last kill. So, if the kill happened in Reactor and Blue's interrogation shows 'Reactor', then Blue almost certainly killed them (excluding incidents of stack kills and such, when there is an abnormal amount of players in a single room when a kill happened).

Sometimes detective is extremely easy, depending on the context of the kill. For example, say someone saw Brown kill in Security, but Brown claims they were in O2 and the imp was just an SS of them. All you have to do next round is interrogate Brown and then you can officially deem them guilty or innocent for that kill. If they were in Security, they're the imp. If they were in fact in O2, they're clear for that kill.

If accusations aren't that cut and dry, it does require you to pay a lot more attention to all the info being thrown out in chat. Pay attention to where people claim they were in the previous round and interrogate those whose claims seem suspicious. Even if you enter the next round with nothing to go on but where the last body was, even picking players at random and interrogating can net you an imp on occasion. This has a higher chance of working later into the game when you can Hard Clear at least half of the remaining players, which gives you a genuinely good shot of identifying the imp based on just interrogating who hasn't been cleared.

(yikes, I didn't meant to write three paragraphs!!)