There are two methods. The first is to use the 'InputForm' function on any of your symbolic expressions.
You could also, in the long run, try to use the defined functions in Mathematica for the symbols to begin with. In my experience, this is a more robust method. For example, the you posted can be represented with the
Sum function. Putting the symbols into Mathematica's Wolfram Documentation (available in the Help menu), will give you how Mathematica interprets these symbols. For example, searching the summation symbol yields the information at the following link: https://reference.wolfram.com/language/ref/character/Sum.html