checking the status of an ongoing "sleep" process from the terminal
The sleep command in itself offers no service that allows the monitoring of the remaining sleep time. It is also not displayed anywhere.

As the sleep command operates without issuing a series of system calls for the kernel, you won't be able to use a typical debugging tool such as dtruss either. If you try to use the lldb debugger to attach to the process, you'll find that the sleep command is also interrupted and cannot be continued.

So it is not as such possible to very precisely determine the amount of time left. If you can make do with an imprecise result, do the following:

First run this command:

ps xa | grep sleep


This should show you the sleep command with the number of seconds that it is supposed to sleep in total. For example:

1234 s0123 S+    0:00.01 sleep 789


Here 789 is the number of seconds that the command is supposed to sleep in total.

Then afterwards run this command:

ps xa -o etime,command -c | grep sleep


It will show you the total time the process has been running:

  03:15 sleep


This means that the process has sleep for 3 minutes and 15 seconds = 195 seconds. The remaining sleep time is 789 - 195 = 594 seconds.