How can I verify the actual wattage of a non-certified PSU?

If the PSU feels light, it’s likely junk. Quality PSUs are generally heavier and come with certifications. The potential damage from a faulty PSU is way too big of a risk to save a few bucks.

A 12V battery load tester could do the trick. You might find one to rent at a local store like Autozone. If you’re comfortable, you can also use a multimeter combined with a clamp meter and a high-wattage resistive load, like a light bulb, to measure wattage. Just keep in mind, if testing seems too risky, it might be wiser to just get a new PSU with a warranty and proper certifications.

You should really think about upgrading to a better PSU. I once had a no-name 750W power supply that failed even when my system wasn't drawing much power. It’s just not worth the gamble with a brand you don’t trust.

Honestly, I don't think there's any reliable software to confirm that on a cheap PSU. The 80 Plus certification isn't the only factor; it's becoming less relevant with newer standards. I'd suggest investing in a quality PSU instead.