If you can't boot, what is the process for narrowing down that your motherboard is the issue? Just replace every single other part and test?

I'm asking because I had a ten-year-old computer die recently and I just bought an entirely new machine because I needed one quickly and didn't have spare parts to test. Like I was suspecting the power supply was keeping it from booting up, but I didn't want to buy a new power supply because if that's not it then now I'm doing a computer-building project with wait times after every part I buy.