I think that the first thing to consider is the age of your windows. If they're more than 20 years old, it might be time for an upgrade. Widows have significantly improved over the years, and new windows are far more energy-efficient. The older your windows are, the bigger the chance that new ones will pay for themselves.
Next, you'll want to look out for signs of visible damage. This includes things like warping, rotting, or cracks in the glass. If the damage is extensive, it might be more cost-effective to replace rather than repair it.
Issues with the operation are another sign. If your windows are difficult to open or close, or if they get stuck, this could be an indication of structural issues that can't be easily fixed.
Finally, if your windows are just not looking their best anymore and are detracting from the overall appearance of your home, replacing them might be the right move. It can add value and curb appeal to your property. Then again, you might want to get them professionally deep-cleaned first. As this Fresh Cleaning Solutions page shows, a good window cleaning Canterbury service can make a big difference.