I think one of the major issues about being a police officer is the honesty of the witness(es) you are dealing with. If I can keep a straight face when I tell a friend a punchline from a witty joke, in an effort to get that friend to think about it before he realises it's a joke and if I can also tell someone to put down the knife and they can still walk away free and after they do they end up in steel bracelets and carted off to custody, then so can anyone else lie to me.
I think it's often a case of going to every job with a fresh mind and giving that person the benifit of the doubt. The training environment would almost certainly suggest that, following up on that, the training environment would also add that we are there to bring cases before the court not decide who we will help or won't because we suspect them of lying, which I would agree is true. I don't think anyone no matter how experienced as a cop can fully work out what most people we deal with think or were thinking or might be thinking. We just have to stick to the facts, the evidence, the past history and our own common sense to operate and function.
This was a lesson which took me all of my probation to learn, many moons ago, one i'm glad I did learn. I would go to jobs and wonder to myself "is this person telling me the truth or is there something more sinister here?" "I think she's not telling me something here" "I think he thinks i'm stupid" etc...Nowadays I don't really keep an opinion on any job I go to, I just think "well this is the job, lets investigate it and stay neutral" just as required to do so. I do my job properly, I help those who need help and I put the bad guy behind bars for the night.
Then I hang up my uniform in the locker room, shower, get changed into my "civvies" and relax to the radio on my way home or paradise depending on how tired I am.
As for those who constantly tell lies and have a history of lying to the police...well they're another story.