To watch the step every say 30 seconds do:
watch -n 30 ls -l
In case you want this to print the differences add --differences flag before the ls command.
According to the OSX man page, there's also
The --cumulative option makes highlighting "sticky", presenting a running display of all positions that have ever changed. The -t or --no-title option turns off the header showing the interval, command, and current time at the top of the display, as well as the following blank line.
Linux/Unix man page can be found here