I hate it, can see some in normal use without 'tests'. I can also see this effect on cars with rear led lights, very distracting!
best laugh today!
Easiest and cheapest way to accurately measure pwm (AFAIK) is this:
To me low pwms are like really fast strobes. With a little bit of training I think I could identify some of my multimode lights on medium just by moving the beam around. I have relatively fast reaction times and am especially good at high speed sports like badminton, table tennis and really fast computer games. So anything that requires fast hand-eye coordination. I wonder if there is a correlation between my reaction times and noticing slow pwms. Same goes low frequencies on old CRT monitors.. I can tell the difference between 60 Hz 72Hz 85Hz above that it gets tricky. I never understood how people worked 8 hours a day on a 60Hz CRT and not notice that.
PWM bugs me when I'm walking with a light. Moving while shinging the light around grass/plants on a low PWM-frequency light is distracting.
haha, thought so myself!
well, i have tried the faucet trick. hmm, yah, looks funny *g* .. but it seems to work fine only with lower frecuency PWM's. the point is, i can flap my arms more vehemently than a typical faucet or shower can unload droplets of water :p
i kick your *utt at ping pong ;) i am learning ma long's backhand J)
One warning I played for 8 years competitively, so as long as your not in a TT club it will be a very, very one-sided match. (read you will be lucky to get 1 return over the net ;-) ) I should still have some now forbidden speed glue laying around.
oké i let you already win hehe 
Legend has it that Ted Williams (famous pro baseball player in case you didn't know) could not watch TV or movies for the same reason - he could see the individual frames...
PWM is also visible on bike-lights in the rain.
It is very easy to show PWM on a photo. This is a Tank 007-E09 versus Fenix LD15 (both in low-mode), at 1/8sec:
I guess the easiest (least resources needed) is the trick that Kreisler mentioned.. You dont need to turn on your camera, take a shower, drive a car, et etc.. Just wave your hands, with the flashlight on.
You can do it anywhere.. it just needs to be dark.
If you really want to check out some strange effects from a low freq. PWM lights try taking a shower with no light on except for the flashlight. I did this once with a P7 light when the power was out during the night. I felt like I had taken some illegal drugs lol.
Take a bit of black cloth and place it over the lens of your lights. If your lights use PWM and you're in a quiet room, you can actually hear the light interacting with the black cloth. I found this trick out with my YEZL Z1X during a power outage.
I learn a lot from this thread, honestly 
Firstly, I did not know much about PWM, and now I know a little about it.
Secondly, from Vectrex, I just google out what is 'speed glue' 
... Vectrex, please be easy on me, as I don't play table tennis 
video most always shows PWM..watching storage wars the other night Dave was cycling through modes on his Icon 2aa light while looking at things in the locker and the camera was doing the rolling..
yep, you often see the rolling thing on youtube videos in flashlight reviews.
The pwm are the times that the led is switched off by the driver each second? So the led is never feeded continuously? I'm confused
The time on the off cycle must be same of the on cycle?
what should be the settings to have best performance(more autonomy,less heat production)?
or is the same? Take an example 1/2s on and 1/2sec off is same of 100 000/200 000s with a total of 0,5s on and 100 000/200 000s off?
I hope you can understand what I wrote, I didn't rereading xD
correcto mundo 
I absolutely hate pwm ...So many people have mentioned the faucet...thats's one main reason it makes me crazy .. it's the reflections off the chrome that sparkles ..Anything shiny tears me up ..maybe If i lived in a matte world I could handle the low pwm .My reaction time is so fast I'd crack the sound barrier on my poor slow friend Vectrex ...gnip gnop
PWM drives me absolutely crazy.
It's the one thing I cannot STAND about 99% of budget lights. They use ridiculously low frequency PWM for all modes below high.
It is why I have my finger hovering over a Zebralight SC600 right now. All modes are current regulated!
Current regulated isn't perfect as it often introduces tint shifting as the emitter is being fed a lower current, but hey! I'd rather have a tint shift than have strobing PWM.
It's the one thing that makes me forget about being frugal with flashlights. I really wish manufacturers would use drivers which (at the very least) are +1000hz PWM.