You cannot (easily).
With azure Web Sites, your site runs in an IIS. RoleEnvironment will always be false, because it is not initialized there. Look at Azure Web Sites more like a regular hosting. Can you tell if your website is running in XYZ Hosting, or in your own IIS?
One thing that you could consider is the Application Settings in your web.config (yes, the appSettings section) and have some setting indicating whether you run on WebSites or not. Read the full article here about App Settings in Azure WebSites to understand what I mean. In short: if you set a setting value via the portal, it will take precedence over what is in web.config. You can even just set it in the Portal, not having it in your web.config.