How can you tell if someone is on your network

@onabreak2 (1162)
United States
March 16, 2007 10:13pm CST
and on your computer. I have a wireless connection and two pc's. So two of them are on wireless while one pc is on cable . Is there anyway I can tell if someone else is using my wireless remotely. I know there has to be some way to do it and even maybe to be alerted when they are. Can anyone suggest anyting to me about this.
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