Most rosetta executables will detect what number of -nstruct they were on, and restart there. Not all executables will. A few do more complex checkpointing; generally only on the BOINC Rosetta@HOME system.
It’s automatic. If you are outputting as PDBs, then you should have a lot of PDBs in the directory already, like (myresult)_0001.pdb, (myresult)_0002.pdb, etc. If you are outputting into a silent file, there will be a file silent.out (or default.out, or something) with a bunch of outputs in it already. Rosetta will see those outputs already exist if you restart it with the same options as before and pick up where it left off.
If you want it to NOT start where it stopped, but instead start over, you’d use the -overwrite flag. That’s the opposite of what you want.