If you’ve solved an equation, put your answer back in as x or y or whatever and make sure the equation works.
If you’ve worked through a proof, take a random line in the middle and check that the left-hand side and right-hand side give you the same value if you substitute in random variables.
Parentheses, Exponents, Multiplication, Division, Addition, and Subtraction. (PEDMAS) rules have to be followed while soling an expression.