There is no way to preview the FONTS on the phone before actual upload to the phone.
I discovered that the best way to tell if the new font will fit properly is by the size of the letters and numbers.
By clicking on the ttf file it will open preview window. Look at the picture below.
To install fonts you have to modify FONTS.XML file and along with FONT.TTF upload it to the phone vis XS++.
Take a look at my post here and some pics.
Create, Edit And Install Fonts For Se K800 - Top Sony Forums