I've played in too many really sucky games, when the DM couldn't get past metagaming themselves. Mindless Skeletons in a 1st level adventure, posted as guards, rushing past frontliners to attack the wizard on the other side of the room, for instance (WTF? did it have a special command to slaughter all non-armoured creatures entering the room?)
Its pathetic.
Intelligent foes may recognise the wizard as such. Depending on the wizard acts, how he is dressed, his mannerisms and such. If the character describes himself as wearing a pointy hat with spell components coming out of his ears, and holding a large musty volume, then by all means kill the weenie. Otherwise, wizards should be played smart. Smart wizards stay alive. Smart wizards tip the battle, just when it is needed most. Because they still have spells available to do so.