Pool service is easy to fake. A truck pulls up, someone nets a few leaves, tips in a scoop of chlorine, and leaves. The pool looks fine for three days and turns on the fourth, and nobody can explain why. The difference between that and real service is a list, and it's worth knowing what's on it.
The physical work comes first. Walls, steps, and the waterline get brushed, because algae takes hold on surfaces long before it colors the water. Debris gets netted off the surface and vacuumed off the floor. The skimmer basket and the pump basket both get emptied, and the filter pressure gets read, because a filter running eight to ten PSI above its clean baseline is choking flow no matter how good the chemistry is.
Then the chemistry, which has to be tested rather than guessed. Free chlorine, pH, total alkalinity, and cyanuric acid all get read, and they interact: high pH makes chlorine sluggish, low alkalinity lets pH swing all over the place, and too much stabilizer locks chlorine up so it reads present on a test but doesn't do much in the water. Dosing one number without looking at the others is how pools stay cloudy while the log book says everything is fine.
On a salt pool there's an extra step, because the salt cell is the chlorine source. Output, salt level, and scale on the plates all matter, and a cell that's scaled or near the end of its life quietly stops producing long before it throws an error.
Finally, you should be told what happened. A reading and a note of what was added, every visit. If the only thing you receive is an invoice, you have no way of knowing whether the water was tested at all, and no record to look back at when something eventually goes wrong.