Since May, Google has combined the storage allowances for Gmail, Drive and Google+/Picasa photos into a single 15GB allocation. It is still possible to track how much storage each service uses — you just need to do a little more work.
As Google Operating System points out, if you click 'manage' at the bottom of the Gmail or Google Drive screen, you'll see a graph showing your total storage usage. If you hover over the graph, you'll see usage broken down by service. In Google Drive, you can also hover over the 'manage' link to see the same details without needing to access a separate screen.
In the example screenshot, Gmail is using everything. Google+ photos hasn't engaged my interest yet. While I use Drive sometimes, all my documents are native, which means they don't count against my storage total. Time to clean out some unwanted large mail, methinks.
The Storage Used by Gmail and Google Drive [Google Operating System]