Width of Text in Entry Field on Screen.

How can you determine the Width of the Text in an Entry Field on the
Screen.

I know the Width of the Entry Field on a screen.  But I want to know
how wide the Text will be.  So if it is 'ABCD' how wide will that be
compared to 'WXY'.
I know it will be different depending on the characters and if they are
upper or lowercase.

In my case I'm using MS Sans Serif 8pt on the screen.
Thanks,

Jim Mumford