A heavier card may indicate re-backing, and a lighter card may indicate a professionally printed fake
and, of course, Johnny, Combo Player (and his shirt has the art for Stifle) Stop That contains Bad Ass Super Secret Tech contains the Power Nine (Ancestral Recall, Black Lotus, Mox Emerald, Mox Jet, Mox Pearl, Mox Ruby, Mox Sapphire, Time Walk, and Timetwister) and Super Secret Tech 7
They usually started with one large set followed by two or three smaller sets of cards and a core set (Ill get to that in a second)
But even in that corner case, you can still activate the Clue since you don't need to tap it to activate it
Not many cards can boast this much power for just 2 Energy