I already deploy my wsp file in my SharePoint site, how can I see the contents of that WSP file?
Just changed the files extension of your wsp from .wsp to .cab and open it.
If you would like to open the file contents of your wsp to visual studio, create a new project in visual studio with "Import SharePoint 2010/2013 Solution Package" project type and choose your wsp to load all its files structure to solution explorer.
OR if you want to make use of the things inside your wsp in your site, it really depends on what you packaged in the wsp and deployed. Lets say your wsp has some files to deploy with the help of module you got to check the files in the target location. Or if your wsp has some visual web parts you got to edit some webpart page in your site and add the web part to the page to see the output.
Let me know if you have any questions