Without buying some expensive gear the best sensing stuff you have are your eyeballs.
I bought some cheap <$3 red safety glasses from china and wanted to ensure they wrre safe,
Easiest way was to set laser power at minimum and view the spot through the goggles, increased power until it was just visible, note the power. I then did the same with my Arc welding goggles (you can buy just the glass cheap too).
I know arc welding goggles are safe for UV .. so did make the assumption that the OD9 rating was fairly relevant to to the laser wavelength and that 100 on the laser power is twice as bright as 50.
the red goggles are between a quarter and a half as good as arc welding glass from memory.
Which puts them in the OD8+ range since 9 is 10 times less light than 8, its a log function.
I am happy to stare at it burning full power with the red goggles on, throw the green ones away, they are terrible, little better than sunglasses, they are designed for red lasers.