I just bought this handsome skill, so I went out... And I saw a huge lvl 20 dragon... I thought it was a boss, but it wasn't. So I asked to many people how I can recognize a boss. They said: "If it glows, it's a boss."
What do they mean by glowing? I never saw a creature glowing... So, I used my Signet of Capture on like every creature I killed, until I just killed 2 bosses. (They weren't glowing...)
Signet of Capture
Woutsie
Neutral Tyrant
Well if you're video card is bad and your game settings are set to low then you won't see the glow. You would just have to see the size of the beast, if it's bigger it's a boss.
Woutsie
Right, setting are on low. Anyway, thanks for the help 
Ketendra
They also have very unusual names too, if you see a mob that looks like the rest of them, but has an odd name, it is probably a boss.