After long experience, Pat Steward, a deputy head from Warwick-shire, says that she can now accurately judge when a small boy is desperate for the toilet. "He'll be holding the front of his trousers twisted into a sort of knot, with a very anxious expression on his face," she says. "I call it 'clutching the crutch'."
It's not always so obvious, of course, and judging when to let a child go and when to say, "hang on please," because a natural break is coming, is a challenge for every primary teacher.
Call it wrongly and you may have an accident on your hands - and, if it's any comfort, every teacher has got it wrong at some time. The usual scenario is that you say, "wait till we've finished this," or "hang on until playtime," whereupon the child has the accident, there's genuine distress and mum comes to complain. "He clearly asked you..."
Liz Haigh, a teacher at Holbrook Primary in Coventry, recalls her own mistake. "It was five minutes before the bell and I asked the boy to wait.
In that short time, he wet himself. The head said she'd have made the same decision and supported me, and it was fine."
Pat believes that you simply have to handle this as part of the job.
"Just be honest with the parent," she says. "After all, it will surely happen at home - with children in cars or at family functions. They have the same judgments to make."
Liz says the introduction into schools of water bottles and fruit is bound to increase the requests for class-time toilet visits.
"And there are never really enough toilets," she adds.
As for me, when I first became a teacher I decided to abdicate from the whole decision. It was the 1960s, after all. "Why should children ask permission to carry out a natural function?" I asked myself. So I told my nine-year-olds they should just go if they wanted.
The policy lasted half a day, during which there was a constant procession of children in and out of a half-full classroom.
So, like every other primary teacher, I went back to one at a time and to learning how to spot the shirkers. You begin, of course, with the ground rules.
Pat says: "With little ones you make sure they go at the beginning of the day, then you give them a chance before play and you insist at lunchtime."
In between times, you just strive to increase your knowledge of your class.
"You get a sixth sense for the ones who are playing up," she says. "There's a look in their eyes."
Liz agrees, but says children just fancy a walk sometimes. "They might be bored from sitting on the carpet too long," she says. "Or perhaps there's something they don't understand and they decide to go for a walk in the hope it'll seem easier when they get back."
Hand on heart, who could be pompous about that?
FOUR TOILET TIPS
* If a child goes frequently, or has accidents, tell the parents and suggest a doctor's visit.
* Make sure all young children bring spare clothing and some plastic bags for wet stuff.
* If an older child inadvertently wets or soils, cover their embarrassment by getting them quietly out of the room without the rest of the class knowing.
* Toilets can be unpleasant, off-putting and a haven for bullying. Listen to children's views and lobby to get things improved for them.