Mobile devices send user agent strings which can be easily identified. If you are using Apache as webserver, the user agent strings are stored in the file agent_log, which is probably stored in the /var/log/httpd directory, depending on your Linux flavor.
Counting the number of matching lines would give a good indication of the current number of mobile visitors. Although not perfect, the following Linux command line should provide you with most of the user agents matching a mobile device:
grep "Opera Mini\|Mobile\|SymbianOS\|SymbOS\|webOS\|BlackBerry" agent_log | wc -l gives the number of hits on your site by mobile browers.
wc -l agent_log gives the total number of hits on your site.