So to ultimately validate you can either use pretty pricey USB power meters that not just display the amperage and voltage, but can record that telemetry, which you can also validate further with clamps and multimeters.
There is a ton you can do to drop Pi power consumption into the ground, which largely is turning off all the hardware on the SBC you aren’t using, followed by undervolting (same clocks, just less power to maintain them), and last by using a very lightweight configuration (look at the Diet Pi distribution) to keep CPU and network interface(s) in the lowest possible power state by just running the absolute minimum you need (and lightweight versions at that) to do the job. After that you could further consider underclocking. Disabling cores can get further tiny gains as well.
Then you could consider purely PXE-booting where you only boot from the SD card to then disable it — relying on keeping everything memory-resident — and having permanent storage be available via network storage presented via ISCSI, or block storage. You’d then only turn on the SD card as part of configuration management changes, to turn it back off after the configuration has finished — or just rebuild the image that’s available via the network then reboot the Pi to start using it. You also then can be picky about what SD card you use, by actually tracking the power consumed accessing particular models — yes, there is a measurable difference.
If I went truly “all in” I’m fairly certain I could have usable 2GB Rev. 1.5 4Bs under 1W of power consumption, with 8GB models under 1.2W, though I’ve been focusing my hobbyist time elsewhere. This isn’t even looking at the Pi Zero 2W, or the Pico, which can be dropped to absolutely absurd low levels of power consumption and do some actual useful things.
I really should do some YouTube videos at some point. I’ve a coworker that’s really starting to ramp up his sub count on his homelab shenanigans, and he says what I’m doing relative to the largest homelabbers blows his mind at times.