Good-quality cotton/elastane mixed fabric is built to last. So when a single hole appears in the side of your underpants, it's an absolute sign you have held onto them for too long. That cotton has been worn through and is long past threadbare.
2. THE FABRIC GOES SEE-THROUGH
Wash after wash, you may start to notice a certain transparency in your fabric. That's the fibres breaking down with wear, and is the precursor to holes. This is actually when I think most underwear should be thrown out - that intermediary stage where the important bits of fabric feel "thinner" than they were when first purchased.
3. THE ELASTIC WAIST BUNCHES
Once the elastic around your waist starts to go - whether it's bunching up, fraying, or otherwise rendering itself useless - the underwear needs to be binned. A baggy waist is, at best, unflattering. At worst, it's downright counterproductive. What's the point at wearing underwear at all if it doesn't hold itself up?
4. THEY'RE TOO TIGHT
Sometimes underpants shrink. Other times they were never the right size in the first place. Or maybe you've even gone up a size since you bought them. No matter the case, tight underpants hurt. Anything that's cutting off your circulation or making them uncomfortable to wear for more than an hour isn't working for you anymore.
5. STAINS
I'm a big advocate for stain removal and general fabric repair before any clothing item needs replaced. It just makes economical sense. Whatever the stain may be on a pair of underpants, if it doesn't come out after a soak in one of those oxy action-style stain removers, they should be thrown out. Whatever the original colour of the stain was, if you keep the pair and continue to wash them, it'll go brown. The dryer will further bake that stain in. Nobody should walk around with that under their clothes.
6. THE BACK IS SAGGY
Ever see an old man in Y-fronts in a movie, and notice they sag at the back? This is something no person, no matter their age, should accept from their underwear. Why exactly some underwear starts to get saggy at the bottom of the bum is beyond me. Don't try to figure it out: just chuck 'em.
7. THEY'RE NOT WHITE ANYMORE
A nice pair of white underpants makes you feel clean and crisp. But when they have absorbed colours from other clothes in the wash (maybe they're slightly pink from a red t-shirt, or slightly blue from jeans), you'll never get that unspoiled feeling back that white should come with.
8. THEY SMELL
This one needs no explanation. If your underwear smells anything but meadow-fresh, get rid of them. Now.
9. THEY RIDE UP
Do you find yourself continually un-doing a wedgie throughout the day? Your underpants aren't the right shape for your bum. There is probably too little fabric in the first place, or they have been cut at an angle not suitable for your body shape. Seriously, nobody needs to feel like their underwear is playing a prank on them.
10. YOU HAVEN'T WORN THEM IN SIX MONTHS
My pair from 2010 clearly wasn't a favourite anymore and were just sitting, unused, in my drawer. Upon another inspection, my underwear drawer actually has several pairs I no longer wear. They're not broken, saggy, or stained, but I just don't really like them anymore. So I'm throwing them all out - and you should do the same for your undies that haven't been worn in six months. They're just taking up valuable dresser real estate.