Hi Emanuele,
That's quite simple. If you have your aligned project, simply go to File > Generate Report and save this pdf file. Open it and go to page 2. Look at the value behind the Ground resolution.
If it for instance gives you a value of 2.0 cm/pix you can calculate the native DEM resolution by multiplying the dense cloud quality levels with the ground resolution.
Ultra high = 1 * 2.0 = DEM resolution of 2 cm
High = 2 * 2.0 = DEM resolution of 4 cm
Medium = 4 * 2.0 = DEM resolution of 8 cm
Low = 8 * 2.0 = DEM resolution of 16 cm
Lowest = 16 * 2.0 = DEM resolution of 32 cm