How to Figure Out a Strum Pattern by Ear
Figuring out a strumming pattern for a specific song by ear sounds like a separate, advanced skill, but it's really just applying things you already know - time signature, accents, muting - to a specific sound, not inventing something new from scratch.
Time signature first, pattern second
Before guessing individual hits, work out the time signature by ear (see the article on 4/4, 3/4, and 6/8) - without it there's no way to know how many beats are in a bar or where to look for accents among them. Most popular music is in 4/4, so that's the hypothesis worth checking first.
Find the accents by ear
Listen not to the guitar as a whole, but specifically for the moment the sound gets louder or 'thicker' within the bar - that's almost always an accented downstroke. It's often easier to hear in the drums or bass of the original recording than in the guitar itself, if there are several instruments playing.
Count the hits per bar
After the accents, count how many individual 'clicks' you can hear in one bar - that tells you whether it's a simple pattern (four even hits) or a denser one, with eighth or sixteenth notes filling the gaps. You don't need to hear every single hit the first time through - getting the overall density right is enough to start.
Check for muting and gaps
If the recording has a distinctive 'chuck' sound or clear short gaps within the bar, it's likely using muted strumming (see the dedicated article) or deliberately skipped hits, not a solid even pattern. This is the part most often missed when working by ear, if you're only tracking the overall tempo.
Assemble the pattern and check it against the recording
Put together a rough draft of the pattern from what you heard, play it along with the original at a slower speed (most players support this), and check it line by line - which hits line up, which don't. Usually after two or three passes like this the pattern lines up closely enough for actual playing, even if it doesn't match the original note for note.
Try it in practice
Open the metronome