How to check if there is a space between two characters?

I'm new to Java programming and I'm now developing a simple application. Now I'm focused on finding a solution on this problem: checking if a String contained in a JTextField contains spaces between each of its characters, like: "1 + 2 - 3 + 7" (which has spaces between every char), but I can't find a solution atm. Could you help me? Thanks in advance ;)