Question with Sqlplus and !

When in Sqlplus you can use the " ! " function to go to the OS,

the question I have is:

When you enable this special mode, and come in the OS, it looks like any normal action in the OS command line.

So, how do you tell the difference ?

That is, without using the EXIT function.

A problem is:

- If you use exit in this mode, you would return to SQLPLUS - that's OK

- But, if you use exit, and you were not in this mode, you'd exit from the OS, and depending on other things, that may mean exiting Putty/telnet