$ ncdump -k mslp.nc
classic
Four possible outputs, if built with --enable-netcdf-4:
classic 64-bit offset netCDF-4 netCDF-4 classic model
If built for netCDF-3 only, can't recognize a netCDF-4 file, so possible outputs are
classic 64-bit offset ncdump: foo.nc: NetCDF: Unknown file format
So even if you won't be using netCDF-4 features soon, it may be useful to install an ncdump that can recognize netCDF-4 files.