It's not out of tune if you don't think it is.
Sadly I have to disagree. I'd say "it's out of tune when I can hear it's out of tune", but then people will think I'm a show-off...
The reality is that accordions go out of tune very slowly, and the player also slowly adjusts to what the accordion sounds like and invariably continues to think that the sound is normal. I know people who played an accordion for 30 years without having it serviced and it was horribly out of tune, yet they thought the instrument was still just fine, like new. (One of them was an accordion teacher...)
To check whether your according is out of tune, without using any measuring device:
1) put it on octave registers: LM, MH, LMH, even LH and play each note for a few seconds (pull and push). There should not be any "beating" (tremolo) to be heard, on any note.
2) put it on a single voice register (L, M, H...) and play octaves. When you play an octave there should not be any "beating" to be heard. All octaves should sound completely dry.
3) put it on MM and start with the standard A (A4=440Hz or thereabout). Listen carefully for the slow frequency of the tremolo you hear. Then go up the scale (and later down). The beating should stay almost the same as you go up (or down) but over many notes you should hear the beating starts getting faster as you go higher up (and lower as you go down). There should not be a very noticeable difference between any adjacent notes, the difference is only noticeable after moving up several notes.
If after all these tests you have not heard anything wrong then either your hearing is shot or your accordion is in tune.
How often should an accordion be tuned? Generally I'd say an accordion needs tuning about every 5 years divided by the number of hours per day it is played. So if you play one hour a day it can use a tune-up at least every 5 years. If you play for 5 hours a day it needs tuning every year. Some professional concert players have their accordion tuned twice a year, others once a year. If you play very little then somewhere between 5 and 10 years is reasonable (an accordion also goes out of tune by not playing at all). Accordions that are still in tune after 20 years do not exist!
A few years ago I played (bass) in a concert of an accordion orchestra where for one piece we had a professional concert accordion player play a solo part. His accordion had been tuned only half a year earlier. I could clearly hear that his accordion was out of tune (on some notes), and I was not the only one who could hear that.