Intro
Killers in game behave different from normal crew members. If you are playing with friends you will notice sometimes your friend becomes too quiet or spoken. They tend to suddenly act nicer than usual. Your friend behavior changes the moment they become an imposter and so do most people. If you play with the same person time and time again you will start to get better at spotting these changes. Here are some I have found most people doing when they become an imposter.

Abnormalities in people
-A person pretends to do a task with visual effects. Gaming room/Medical Exam
-Stand near a task and takes longer than what it would typically take to accomplish. Especially if you know your friend is experienced. Especially if the task bar doesn't move.
-You find them doing the same task twice and it's not a repeatable one.
-Standing near vents.
-You find the person following/walking around more than doing tasks. You will see them walking around a room twice. Walking around the map with no interest in doing tasks.
-They straight out go to the farthest room/tasks at the start of the game. Most tasks at the start of the game tend to be closer.
-They are around the area of the dead person.
-Not around people(no alibis).
-Around people that have died. Keep track of people traveling together.
-They go around rooms just to look if people are in it rather than to do a task.
-They wonder more than anything.
-Person coming off a room you previously checked/knew it was empty.
-A report happens too quick.
-A person with these traits reporting a crime
-Voting during a skip
-Voting the least suspect
-Not voting the obvious killer

Staying safe
I recommend you don't put priority in doing tasks. You will most often be killed that way. Instead, pretend to do task while there's someone near you and look for suspicious behavior. Only do tasks when you are alone.
-Keep a distance from people. Treat this like corona virus. Practice Social distancing.
-once someone enters a room you are in try and leave. Go around the person. Consider them a suspect. Especially if they enter the room and leave or you successfully leave the area.

Finally have a suspect?
The more and more of these signs are display the more of a suspect they become. Once you noticed the person displayed many of these suspicious behavior them consider them a suspect. These are further tips in catching them on the act once you got a suspect.

Baits
Killing
1) Be with 2 people.
2) Leave the room and leave for a few seconds.
3) Come back and see if someone died.

Venting
1)Follow your suspect. Pretend you aren't following them by keeping a distance/entering another room. Wait until they enter a room and stay near the exit of that room. If they disappear after a while without leaving the room they had just enter then they vented.

Reporting the suspect
Go to the alert room(always at the start of each round) or report the body. It's good to let people know if you suspect someone and write why.

Hope these tips make you a pro detective. I wont be responsible if you kill innocents because of this guide.