I am not someone that plays by ear, but generally speaking, I can spot them a mile away by their lack of technique. The best self taught player I ever saw only used 3 out of 5 fingers. He was good but so limited.
Now, no one please infer that I meant that all people self taught have bad technique, it is inherent in that you will excel in the area you practice the most. People playing by ear generally don’t practice scales, arpeggios, Hanon exercises, but they do develop an amazing ear and usually quite the repertoire.
The OP has "folk"' in their moniker and here has asked about "tunes," a conventional term for folk music of Irish and other genres. Based on these indicators, I'm hypothesizing that the question concerns largely treble-side single-melody-line folk tunes learned by ear.
In many folk genres, not only is ear learning and playing not a signal of poor technique, it is considered poor musicianship not to do so. One might use "the dots" at times, but learning and playing by ear is almost de rigueur. At workshops taught by master musicians at trad festivals or "camps," the tunes are more often than not taught by ear, usually phrase by phrase, and you are expected to have a recording device with you. People also acquire and build repertoire by recording tunes at sessions and learning them later from the recording.
At the same time, though, it's true that fingering folk tunes can be a challenge at first for apprentice-level accordionists to figure out on their own, whether by ear or by written music lacking fingering directions. I do find this especially so for PA as opposed to button instruments be they unisonoric (CBA) or bisonoric---Am I recalling correctly from other threads that the OP plays PA?
At first, fingering folk tunes on your own on a PA can seem maddeningly tune-specific, but there is knack to it that comes with time and practice at working it out. Basically, if your fingers tangle up or you run out of fingers to get you where you need to go, you have to go back and try again until you figure out the "solution" that works for playing it smoothly and to speed.
The good news is that a "solution" always exists, even if it means slightly rearranging (or deleting) nonessential notes to make the tune finger in an "instrument-friendly" way to your instrument. Remember, that folk tune is not a classical score by Lizst or Beethoven drafted with a quill and inkstand for the piano. It's a snatch of melody transmitted aurally and learned by ear and fingered by generations of fiddlers, flutists, pipers, etc. They are all figuring it out just like you are.
Part of it getting it on PA does involve classic techniques for getting up and down a keyboard, and some instruction can be helpful, as posters have noted above. There are also techniques that seem more folk/trad related-- for example, if you watch Irish and Scottish PA players you'll see little to no pinky use. This differs from conservatory technique, but for whatever reason it does help with working out PA fingering for folk tunes. My own facility at tune fingering grew by leaps and bounds after I designated the fifth finger "emergency use only."
It can be very helpful to have at least a few Zoom meetings with one of the awesome trad PA players who teach, and can take you through the process of fingering some tunes.
