>It's always impossible to tell, for any given sequence, whether it was produced by a fair die.
If the sequence is long enough you can model how likely it is to have been produced by a fair die. Are all numbers equally distributed? Are some numbers more likely to follow or not follow other numbers? Are some patterns repeating?
Of course any sequence can be produced by a fair die, but you can still create some objective metric that will tell you how truly random a sequence is, and the longer it is the more accurate it will be. It's what tests like Diehard are about after all.
Can you roll a fair die a thousand times and only get 6s? Well yes of course. It'll never happen though.