In addition, most (but not all) Unicode-encoded Arabic fonts are available in the more modern OTF font format that will eventually replace the older TTF and PS font formats.
Validating a font
To check if a font is a properly Unicode-encoded Arabic font, you can open it in the character selection dialog, and select the Arabic code page, where you should see Arabic characters.
You can also check that the Arabic characters are in the correct position by comparing the layout using your font to the layout obtained from using the Arial Unicode MS reference font.
Note that not all Arabic fonts will have all the officially defined ligatures of the Arabic language.
The better fonts will have them all, or almost, but the cheaper, or free, ones may only have the most basic ligatures (such as lam-aleh).