Someone just gave a good answer to a similar question
on the perl.beginners list today. It was how to detect jpg files,
but the method should work for you

#!/usr/bin/perl
#all .gif, .jpg and .png file have a special header id that identify t
+heir 
#types. for example, starting from the 7th byte of a JPEG file, you sh
+ould 
#see the special id as: ox4A 0x46 0x49 0x46 and 0x00, the following ch
+ecks 
#for that: 

my $t = undef;
my($one,$two,$three,$four,$five);

open(IMG,"$ARGV[0]") || die $!;
sysread(IMG,$t,6); #-- discard the first 6 bytes 

#-- read the next five bytes 
sysread(IMG,$one,1);
sysread(IMG,$two,1);
sysread(IMG,$three,1);
sysread(IMG,$four,1);
sysread(IMG,$five,1);

#-- unpack them and make sure they are the right magic ID 
#-- you can get the id from the JPEG specification 
if(unpack("C",$one) == 74 && unpack("C",$two) == 70 &&
unpack("C",$three) == 73 && unpack("C",$four) == 70 &&
unpack("C",$five) == 0){
print "jpeg\n";
}else{
print "not jpeg\n"
}
close(IMG);

#you can do pretty much the same thing to png file. just go to google,
+ search 
#for png specification, find out what the maigc id should look like an
+d just 
#code it according to the specfication.
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