to prove a game is beatable

what is requirement/criteria to prove if a game is beatable ?
suppose back to 100 years ago, no computer,
a guy played with perfect BS, shuffled evry hand, var 1.27,
no idea of H.E.
how many hands he had to play and won [net win] to prove it ??
let's say
+100 units/1000 hands,
+1000 units/ 10,000 hands,.... good enough ? where to draw a line ?

for academic disscussion and brain exercise.