You can simulate responsive design by resizing your browser window and see how your project resizes based on the browser window size. However, you should still test on the actual devices that your user base will be using. You can easily find out this information by looking at the site's current analytics and seeing what their users are coming to their site with. You can also use a tool available at http://caniuse.com/usage_table.php to see which browser versions are popular. If they come to the site with a device you do not have, you might want to think strongly about getting that device. Personally, I have found that even device simulators are not always accurate about what happens on the actual device. So, the following is a list of devices that your project should be tested on:
iPhone 4/4s – this is more for load times and site speed
iPhone 5
An Android device running Android 2.3
An Android device running Android 4.0 or higher
iPad
Nexus 7