I assume you are using windows. Open the command prompt and type ipconfig and find out your local address (on your pc) it should look something like 192.168.1.13 or 192.168.0.5 where the end digit is the one that changes. It should be next to IPv4 Address.
If your WAMP does not use virtual hosts the next step is to enter that IP address on your phones browser ie http://192.168.1.13 If you have a virtual host then you will need root to edit the hosts file.
If you want to test the responsiveness / mobile design of your website you can change your user agent in chrome or other browsers to mimic a mobile.
See http://googlesystem.blogspot.co.uk/2011/12/changing-user-agent-new-google-chrome.html.
Edit: Chrome dev tools now has a mobile debug tool where you can change the size of the viewport, spoof user agents, connections (4G, 3G etc).
If you get forbidden access then see this question WAMP error: Forbidden You don't have permission to access /phpmyadmin/ on this server. Basically, change the occurrances of deny,allow to allow,deny in the httpd.conf file. You can access this by the WAMP menu.
To eliminate possible causes of the issue for now set your config file to
<Directory />
Options FollowSymLinks
AllowOverride All
Order allow,deny
Allow from all
<RequireAll>
Require all granted
</RequireAll>
</Directory>
As thatis working for my windows PC, if you have the directory config block as well change that also to allow all.
Config file that fixed the problem:
https://gist.github.com/samvaughton/6790739
Problem was that the /www apache directory config block still had deny set as default and only allowed from localhost.