How To I Find Whether A File With A Specified Path Is Exists In Client System Or Not ?

You can't in practice: Server systems do not have direct access to client hardware - they have to go via the browser.

While it is in theory possible, in practice it won't work: In order to do it, your client will have to be running IE. No other browser will let you do it. Then, IE has to be configured to allow you to send it an ActiveX control (and this is disabled by default). Then IE has to run the ActiveX control (and this is set to "get permissions from the user first, and warn him that this is potentially a dangerous thing to do" by default).
And then...you can write the ActiveX control.

Even if you could find out, it wouldn't do you any good as you can't access the file to do anything with it from the server anyway.

Think about it: if you had the access you'd need, so would everyone else - including malicious sites...