It is as simple as grabbing an Ohm-Meter and measuring across the boiler elements.
A couple of things --
The Overheat T-Stat may have opened -- they are supposed to reclose after the overheat condition - this may have not.
Without looking at my diagrams, if either the BREW t-stat or STEAM t-stat opened and stayed open, you would have the same situation.
The gran is a great little machine and is easy to fix once you know what is up.
By the by, the heating elements are on the OUTSIDE Of the boiler with most gaggias -- so they can heat air all day long and never worry.
Additionally - boiler elements on the outside do not corrode as fast as others.