1. Launch any Valve multiplayer game
2. Click Options
3. Select the Keyboard tab
4. Click the Advanced button
5. Check the box labeled Enable Developer Console
6. Click Apply and then click OK
7. Join or create a server
8. From the menu screen, press the tilde ~ key (typically in the upper left hand corner of the keyboard) to load the Console
9. Type status into the console text field and press the Enter key
10. Your SteamID will be displayed next to your nickname in the console window
SteamID example: STEAM_0:1:1234567
Källa: http://supportwiki.steampowered.com/wiki/SteamID