I don't know how many times I visit friends' places or go to parties and their stereo speakers are out of phase. Even worse - they don't even seem notice how horrible they sound.
They're probably so used to it that they think that's what the system's MEANT to sound like.
As an audio engineer, I can usually tell straight away and it drives me crazy. Usually so crazy I can't even even pay attention to what people are saying or even enjoy my drink until I fix it.
How can you tell the speakers are out of phase?
There's a distinct lack of bass frequencies (that's the low rumbly ones), unless you're close to one speaker only.
When you walk across from one speaker to the other, parts of the song seem to follow you, or "swim".
It feels like there's a "hole" in the sound in between the two speakers.
Here's the easy test.
Move both of the speakers together, side by side. Is there more or less bass?
If there's more bottom-end, then it's usually all good.*
If the sound gets thin and harsh, then your speakers are out of phase.
How to fix it? Easy.
1) Turn OFF your stereo.
2) On the back of ONE of the speakers swap the two wires.
3) Power up and enjoy a better sound!
*Sometimes, on a Friday at five minutes before the end of work, an apprentice speaker assembler can solder the wires around the wrong way on only ONE of the speakers in your speaker box. This makes it a nightmare to figure out what's wrong, and takes some higher-tech equipment to analyse.