O -we bought a Man U one after they won the European Cup -(co-incedence or what for tonight) -cant stand them at all but H bought it -we checked feedback et al -but really we have no idea if it is authentic.
I daresay the teams will sign hundreds after a big game and people or charities will go on to sell them -we received a certificate of authenticity but its behind the framed shirt so cant check.
Suppose the only way you could be 100% sure is you were there when they signed it.
I would go by the feedback if its on EBay -other than that you cant be sure.