Recognize end of Input in a Turing Machine
There are many possible input conventions for Turing machines. The one I know has the tape initialized by the input word (in case of a single input) surrounded by blanks, where a blank is a symbol of the tape alphabet that isn't in the input alphabet. The Turing machine treats all tape symbols in the same way, so you can do with them whatever you want.

Other conventions are also possible. For example, we might want to forbid writing blank for some reason. Only you know which convention is used in your class. The various input conventions are mostly equivalent, so in practice it usually doesn't matter which one is used.