Hope this is a known one. New double glazing should be labelled, shouldn't it? But rather than getting new windows, are my uPVC windows fine? What checks? To elaborate - what should I look at on the windows as a means of checking they are adequately energy efficient so I don't need to buy replacements.
Hire someone that can do an energy audit. Check with your local utilities as they may even offer rebates on the cost of that. They'll use a Thermal Imaging Camera that can answer this question for you.
It's not so much whether or not your windows are a problem as much as it is the bigger question "what parts of your house are the problem". Even if your windows aren't efficient, you may have bigger issues elsewhere that will give you a bigger ROI in terms of energy costs than replacing the windows.
Read my answer on the following question -
This method works really really well for windows because of the temperature differences. Also if you do this in the middle of the summer/winter it would be optimal. Other thing you can do too for windows is take a temperature reading in the middle of the window and compare it to the temperature along the window edges (winter works best). You would be measuring how well your windows are sealed, how well the windows fit in the frame, and the caulk job but that is important too.
Have you checked your windows for any weak spots? a professional can measure the amount of sunlight passing through the panes, and this can lead to potential treatments or simply being told the windows are fine. To Repair or replace if you do find faults becomes the dilemma? Down to costs and as mentioned above, replacement uPVC can be very expensive. If you look after your uPVC windows then they should be fine. Look for signs of improvement being needed. Rot won't be an issue as you have uPVC though do check for worn out cranks and foggy double pane glass. Essentially, professionally placed glass should be maximising solar heat during winter and minimising in Summer. The most energy efficient glass for double glazing is low emissivity (Low-E) glass. It limits the amount of heat escaping through windows. Have your windows checked so you can have peace of mind.
Most window/door installation companies offer an efficiency inspection, that will not only check the design of your windows, but will also check the seals, fitment, and functionality of your doors and windows as well. Be careful, as some less reputable companies will use this as an opportunity to sell you things you don't necessarily need. My source, is that I have owned and operated a window/door installation business for over 25 years.