If you're in Lite mode, you'll see a box in the bottom left with a lightning bolt and this message: "You're in Lite mode." You can click: "Switch back to full Maps" if your browser supports it.
The Lite interface uses a hamburger-style menu, just like the mobile apps.
For now, the old Google Maps is still available if you use this link: https://www.google.com/maps/mms, but there's a message which says that "this version of Google Maps is updating soon".
Update (June 25, 2015): Classic Google Maps is no longer available.