depends whether you want the theoretical or practical answer.
theoretically, the outcome of the next spin is predicable IF you assume no friction, the clock fan spins at exactly the same constant speed every time, the reel index arms instantly and perfectly fell into an index disc slot when released, and the reel kick force was exactly the same every time.
in practice, none of those assumptions are true so you get some variation from ricks "pull mechanics".
however, relying on small random variations wasn't good enough, so the variator ratchets david talked about are added to change the amount of time the reels spin before the first reel indexes, and the amount of time before any reel with a variator link subsequently indexes.
'course, if you know the position of the variators and it was a perfect world, the spin outcome would still be predictable, but the only way that could help you is on a multi-coin game where you'd bet one coin until you knew the next spin was a winner then bet max coins ... or you'd have to track someone else's play then convince them to leave when a winner was imminent.
some early mechanical slots were subject to "rhythm play" and handle manipulation ... but I've never seen a good technical description of what those issues were.