4/4, 3/4, and 6/8 in Practice
A time signature on paper is just two numbers, but in practice it's what actually decides where the hand swings and where the accent lands. The difference between 4/4, 3/4, and 6/8 is audible and physical once you work through them as a count out loud on the guitar, not as theory.
4/4 - four even beats
The most common time signature in popular music: four equal beats per bar, counted '1-2-3-4'. The accent naturally falls on beat one - that's the easiest beat to hear in any familiar song if you count bars off it.
3/4 - the waltz count
Three beats per bar instead of four, counted '1-2-3' with the accent on beat one - a distinctive sway that's instantly recognizable by ear (the classic example is a waltz). On guitar, 3/4 often gets confused with 4/4 if you strum on autopilot in even fours - the difference becomes clear once you deliberately count to three instead of four.
6/8 - six beats that feel like two
6/8 is formally counted in six, but by ear and in the hands it reads as two large groups of three - '1-2-3-4-5-6' with accents on 1 and 4. That feeling of 'two big pushes, not six small ones' is what distinguishes 6/8 from simply counting six beats at a 3/4-like tempo.
How to hear the time signature in an unfamiliar song
The easiest way is to find the beat your foot wants to tap without thinking - that's almost always beat one. Then count from one 'one' to the next: if you land on four, it's likely 4/4; on three, 3/4; if it feels like two big pushes with a small sway inside each, it's probably 6/8.
Why this actually matters in practice
The time signature decides where a strumming pattern naturally puts its accent and where a chord change logically fits (see the article on changing chords inside a strumming pattern) - a pattern that sounds great in 4/4 almost never transfers directly to 3/4 or 6/8 unchanged, precisely because the accents and the hand's anchor points sit in different places within the bar.
Try it in practice
Open the metronome