Walk a casino floor and almost every machine in front of you is telling you nothing. The reels landed where they landed, the last player left, and the next spin starts from a clean slate. That is how most slot machines work, and no amount of watching will change it.
A minority of machines are different. They carry a number forward. Something the previous player built up is still sitting on the glass when they walk away, and it is still there when you sit down. That carried number is what advantage players mean by machine state, and it is the only thing on a casino floor you can genuinely read.
A progressive meter that has climbed since its last reset, particularly one with a stated ceiling it cannot pass. An accumulator or counter: a running tally of symbols, coins, heads, orbs or stamps that survives the end of a session. Board or collection progress, where squares are filled, a map is partly walked, or a meter sits part-charged toward a feature. And bank-level progress, shared across a row of linked cabinets and shown on the overhead signage.
What does not count
Everything people usually watch. How long it has been since the machine last paid. Whether it feels hot or cold. Whether the person before you lost heavily. Where it sits relative to the door. None of that persists in any readable form, because none of it is stored anywhere you can see. If a number is not printed on the machine, it is not state. It is a story.
Walk a bank once at a normal pace, looking only at the glass above and around the reels. You are not evaluating machines yet; you are sorting them into carries a number and does not. Almost all of them land in the second pile, and that is the point. The walk is a filter, not an assessment.
Second pass, only on the ones that carry a number: read the value, read the ceiling if the game states one, and note which bet level and denomination the reading belongs to. The same cabinet often runs a separate meter for each, so a number read at the wrong bet level is worse than no number at all.
The physical locations are consistent enough to learn once: the top box, the upper band of the main screen, inside the game window, the overhead bank sign, and the help screen. We have mapped where each type sits, and how to step through every bet level without paying for spins, in How to Read Slot Machine Meters.
Reading the number is the skill you can practise for free, on any floor, starting tonight. Deciding what that number is worth is a separate question, answered machine by machine. For a worked example on a machine most people have already played, see the Buffalo Link breakdown, which is free to read.