Only if someone lets you know it was used, or if you try the code yourself and it says invalid code or all ready used.
but if i try a valid code will i still be able to say i don't want to redeem it?
Nope, once you try it, it is used... sadly. (edit: unless the person that is trying the code all ready owns the product, can the code still be used.... shame that if you use a code for a product you all ready own, it will still NOT let you know if it is redeemed or not)
Basically its blind faith for steam codes.