The whole shell won't lift - just separate scutes.
Here's a thread with some good photos of a shell shedding:
http://www.redearslider.com/forum/viewt ... ht=lifting
About half way down are a couple of links to photos of a shedding turtle. That's about as clean a shed as you're ever going to see. Those scutes are just falling off. If you gave them a little tug, off they'd come. You may not ever see such a clean shed.
The second set of pictures towards the bottom is more typical in my experience. The shell is getting sort of pale and opaque looking and you see a little lifting around the edges of a few scutes.
And there are still other ways that shedding can manifest itself. Sometimes you can see through the clear surface of the scute that there's water underneath, and when you press gently, water may ooze out.
Also, when the turtle is under water, sometimes it looks like there are metalic or coppery patches on the shell. This is air trapped underneath the scutes that are getting ready to drop off.