There are two ways:
One:
While in-game open the console using the ~ key and type status in the console.
Or other:
Locate your steam directory on your hard drive, usually C:\Program Files\Steam -- find steam.log file and open it with notepad. Look for lines such as: Found existing connection HLSW 84.200.252.204:27015 for 0:0:2222222. The last few numbers is your steam id, ie: 0:0:2222222.