How can I know if someone has come by my password...?

and is accessing my account? Are there ny indicators to tell me this and if my password has been hacked, how do I go about changing it.

I keep getting a flag advising that I have been signed from my computer because I have signed on at a remote terminal. Does this indicate that someone else has signed on under my password and is

hacking my account? any assistance would be appreciated. Thanks!