Contact sports are some of the riskiest
Spotting athletes who are still unfit to return to action days or weeks after a blow to the head could be made simpler by a new test.
The check, devised by University of Michigan scientists, looks for sluggish reaction times.
Those too slow to catch a falling object - a weighted cylinder - are likely to have concussion.
A UK expert warned athletes should expect a three-week layoff after concussion.
Minor head injuries are part and parcel of many of the most popular sports in the UK, particularly contact sports such as rugby.
Many of these concussions go undetected by the player or their coaches.
And even when spotted, there is some uncertainty as to when the player should be allowed to compete again.
The after-effects can linger for several days, even after other more obvious symptoms such as headaches, dizziness and confusion have abated.
The Michigan test involved 209 young male and female footballers and wrestlers.
They were asked to catch the weighted cylinder when it was dropped by the coach, and their speed of response was recorded.
Then, during the season, if any of them suffered a concussion, the test was repeated a few days afterwards.
Seven of the athletes had a longer reaction time - on average 15% longer.
Dr James Eckner, who devised the test, said: "Because of its simplicity and low cost, this test may work well with youth athletes, where there is limited access to computerised testing of reaction time."
He will showcase his work at the American Academy of Neurology's 62nd annual meeting in Toronto in April.
Luke Griggs, from the head injury charity Headway, said that while the findings were interesting, more research would have to be carried out to confirm them.
He added that anyone displaying the signs of concussion should be taken to hospital.
He said: "There are a number of symptoms to look out for when attempting to spot concussion, including blurred vision, disorientation and the inability to concentrate.
"If these symptoms are recognised in a player, it is vital they are removed from the field of play and taken to the nearest accident and emergency ward for proper medical assessment.
"They should then refrain from playing any contact sport for at least three weeks unless told otherwise by a doctor."