So there is of course a way to do this - you have to change the Partial Structure Display setting to show the load bearing structure for example and then add the elevation, nice and easy.
But - this is where Graphisoft overthought this in my opinion - they get hidden when you switch back. So if you want to use this, you have to overlay two plans with different PSD settings on a layout.
The other way which we use, is to place a label (AUT Bauteilhöhe, INT Building Elemnt Height?). They have the ability to show the right elevation, but only for slabs. And you have to add a symbol separately.
There is a way to do this, it requires a few things to work:
you need to set up 'core' and finish' layers properly in your composite
you need to set the reference plane of the slab to the top of the core
now you can use a text label and add autotext to show the dimension to either 'top to project zero' or 'elevation to project zero'. Top will show the finished slab and the other option shows the level of your reference plane