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.