How can I tell if he is online (loged in ) from another location.
He uses net zero I use Yahoo or comcast. We are at separate locations different computers

Who? Different location on the same screen name? You really need to be more specific...

You can't tell in general if someone is online (well, if he has a fixed IP address you can ping the address, but that will tell you his computer is on the internet, not that he's personally online). But you both have instant messaging accounts (e.g. AIM), and you both keep your status up to date, you'll be able to open the instant messaging client and see whether your friend is online. Note that if he's always connected to the internet and he doesn't update his IM status to Away, this won't give you good information.

By the way, if the goal is communication with each other, you might want to consider Skype or Gizmo for free internet phone calls (you both need the client software to be able to do this for free):

Skype - internet calls - download Skype free now - free calls - official website (http://www.skype.com/)

Gizmo – A free phone for your computer (http://www.gizmoproject.com/)

Vasily

Vasily

How can you know what? Your title is not descriptive enough, but I shall try and help you.