A warrant on/involving what?
Generally, you'll want to look at the market value of what underlies the warrant and the conditions/terms for converting the warrant to it's underlying instrument(s). But that may not give you a "simple" answer to the "fair value" (markets are funny that way! See below for my question on what you mean by "fair value.")
Here's something to chew on if you want to get complicated (it might be easier if you can give some more details about the warrant you have in mind when asking your question):
http://stocks.rwe.onvista.de/encyclopedia.html?ID_DEFINITION=501&LETTER_START=I&LANG=en
Also, are you trying to determine "fair value" for some specific tax/legal reason, or are you just trying to get an idea about what a specific warrant is worth?