Ideally, you should slowly close in on them from the other side of the room and if you hear the sound of laughter, do not be fooled. This could be the faked kind that's put in afterwards.
At least the BBC's latest one, Big School (BBC One, Friday, 9pm) is promising. It is written by and stars David Walliams. It also has Catherine Tate in it. And Frances de la Tour. And it has a big supporting cast of young actors who are quite brilliant as the pupils. They don't say much. They don't do much. And they are unimpressed by pathetic attempts to be enthusiastic or authoritative. Having spent an afternoon talking to pupils recently about journalism, I can confirm this is an accurate representation of what they are like.
Walliams is also pretty accurate as a teacher, or rather a very particular type of teacher: the gauche type who probably hasn't had much, if any, sex; the socially useless type who doesn't switch his phone on over the weekend because he's not expecting any calls; the man who teaches children because he is essentially a child himself.
The character is a cliche - in fact, pretty much all the teacher characters are cliches, particularly the PE teacher (played by Philip Glenister) prowling round the school like a mongrel dog - and this is because it is hard to say anything new about schools in comedy or drama. Even as I was smiling wanly at a slightly amusing line in the first episode, I was thinking: couldn't Walliams have chosen a more original scenario? His last series, Come Fly With Me, was great partly because no-one had ever done a comedy set in an airport before. Everybody's done a comedy set in a school.
The best characters in the first episode of Big School were the women, particularly Tate as the French teacher Miss Postern and de la Tour as the headmistress Ms Baron.
With Miss Postern, Tate finds herself in the interesting situation of looking back at herself. For years, she was famous for playing Lauren, the mouthy pupil who kept asking "Am I bovvered?" Now here she is playing a teacher facing a classroom full of mouthy pupils, one of whom did the am-I-bovvered face to her. It felt like comedy revenge.
Frances de la Tour's character was also delightful and had most of the best lines, as if she demanded them in her contract (perhaps she did). "May I sit down," asks Miss Postern. "I'd rather you didn't," says Ms Baron. "I've just had the chairs upholstered."
Although overshadowed by the women, Walliams's character is perfectly fine. A big problem is that he has reached that stage in his career where you always see him looking out of the men and women he plays.
In the early days, before he was famous, he could switch characters with ease. Now all you see is David Walliams, the straight man who's little bit camp, mugging madly. In the end, sadly, it will be that public persona, created consciously or not, that will spoil Big School and make us turn to the next new sitcom instead.