Yeah, that's the difficult part. The article in the link above, that guy also uses a laptop with a dedicated soundcard, plus software to measure the PWM frequency. This setup is not very portable. I was thinking of having a small pocktable oscilloscope that I can bring to the store with me to check if a laptop model has PWM or not. If the setup is too big, it would not be practical to bring to the store to quickly test if a laptop has PWM or not.
But the portable oscilloscope from ebay in the link above can detect frequency, somewhat similar to laptopmedia.com's PWM testing chart. Is it possible to just attach a photo diode to the oscilloscope just like that? I google's photo diode for oscilloscope, and cant seem to find any. Maybe like you said, need to manually solder it, which if that is the only way, I will not be able to do.
If someone finds a photo diode that can be connected to a portable oscilloscope like in the link above, do let me know.