Assuming you're setup for outside access (ie, it works from the inside with an external proxyserver)..
Put the site's domainname and IP-address in your hosts-file on your workstation(s) (/etc/hosts or \windows\system32\drivers\etc\hosts). Restart browser and you should be able to browse them without the external proxy.
The alternative is using some tricky split-view DNS, but that's probably also overkill
Oh, and BTW, an even better solution would be to put the webserver in a DMZ, with it's public IP, protected with a firewall and another firewall to your internal network. That way it's transparent to you and safe as it should be..