Almost every slot machine on a casino floor is unreadable. Outcomes are drawn fresh on every spin, nothing carries over, and no amount of watching tells you what happens next. A small minority of machines are different, and they are different for a structural reason: something on them persists between players. That persistent thing is what an advantage player reads.
There are three families worth being able to recognise on sight. Most of the value in scouting a floor comes from telling them apart quickly and correctly.
Must-hit-by progressives
A must-hit-by carries a published ceiling it cannot pass. The counter climbs as people play, drops back to a floor after a hit, and somewhere between the two the jackpot is forced. The read is a single comparison: how close is the current counter to the ceiling.
This is the best family to start with, because the read is identical on every title. You learn one calculation and reuse it everywhere. The homework you do per machine is the reset floor, the ceiling, and how fast the counter moves at the bet level you would actually play.
What to look for: a printed "must hit by" value, or a jackpot label stating a maximum. It is usually written on the glass.
Accumulators
An accumulator banks progress toward a bonus, and that progress does not reset when a player cashes out. Collected symbols, filled discs, orbs, a coin bank, a running tally: whatever the theme calls it, somebody else paid for the progress sitting there and then walked away.
The read is different in kind. Instead of measuring distance to a ceiling, you estimate what it costs to cover the remaining distance and compare that to what the bonus is worth. If finishing costs less than collecting pays, it is a play.
What to look for: a collection display that is partly filled while nobody is sitting there. Progress plus an empty seat means somebody abandoned it partway.
Mystery progressives
A mystery jackpot fires at a randomly drawn value with no published ceiling. From ten feet away this looks like a must-hit-by, and it is not the same thing at all. Without a stated maximum there is no distance to measure, so a high counter tells you far less than it appears to.
What to look for: a progressive display with no "must hit by" text anywhere on it. Read the glass before you assume.
Telling them apart in two questions
First: does anything on this machine persist when the player leaves? If not, walk. It is an ordinary slot and there is nothing to read.
Second: is that persistent thing bounded by a published maximum? If it is, you are in must-hit-by territory and the read is the gap to the ceiling. If it is not, you are looking at either an accumulator, where the read is cost-to-finish, or a mystery progressive, where the honest answer is often that you cannot price it at all.
The rule that applies to all three
Whatever the mechanic, you sat down because of a specific condition, and when that condition ends the machine reverts to an ordinary negative-expectation slot. The must-hit-by fires and resets to its floor. The accumulator bank empties. The seat that was worth taking five minutes ago is worth nothing now.
Standing up on time is not a footnote, it is most of the discipline. A correct read followed by an hour of extra play is a losing session with a good story attached.
One thing worth saying plainly: an edge is a long-run statement about a decision, not a promise about the next hour. You can read a machine correctly and still lose the session. Size what you bring accordingly.
Go deeper
Run the Slots publishes free explainers on each of these mechanics, plus one machine worked end to end:
must-hit-by progressives explained