When a job has started output generated by the jobscript will be written into the job outputfiles.
If you haven't changed the names of these files using the sbatch parameters –output or –errror, these files will be called slurm-<jobid>.out and slurm-<jobid>.err.
<jobid> will be replaced by the actual job id of the jobs. The .out file contains everything written to standard output, and the .err file will contain messages that were written to standard error. The latter will normally only appear in case of error messages.
Note that output will be written to these files as soon as the job starts.