|how to know the person accessing my web page is accessing from mobile or computer?|
You don't and as far as I can see, short of asking them (via a web link they could click on) there's no way to find out.
I'd also like to point out that whether the device they're using is a desktop, a laptop, or a netbook, it's still a "computer"
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.