How do i determined my foundation color at a drugstore?

i noe that you can try on the product like how you can at the malls or other beauty stores like mac sehpora channel etc.. so how do i determined my foundation type or color i should say.. because i heard a saying that you can look on a chart that they provide you and wat turns visble on your wrist then is your skin type but i also herad that it dosnt work too. so how do i determined my foundation color then.. please help me…!!