I think if it looks dark form the outside it's probably too tinted.
This thread has some comparisons http://www.babybmw.net/forum/viewtopic.php?t=89567
http://uploads.tapatalk-cdn.com/2016...19a40e4840.jpg

The back 3 windows are probably factory tinted and the previous owner probably added the front tints. I don't think BMW ever did tinted front and windscreens in Europe.