Radouani,you can't but that's only my opinion.If buying a good racer the vender should have prize cards for the bird,young birds need to be raced.How often do we pick out a "super young bird" only for it to fall at the first hurdle.When I pick up my birds I check the body,wing,eye and throat but this is to see the birds have no problems,there are many theories but for me the basket doesn't lie.Having said all that,there are genuine fanciers who have had their families of birds many years and know them inside out and can usually pick a decent bird out with a good chance it will make the grade.