1. Look at the objects on either side of the operator.
2. Ask yourself what you think those objects are (scalar, vector, matrix, symbolic, whatever).
3. Ask MATLAB what it thinks those objects are. If you're not sure, then go into interactive mode, and execute all lines up to the current line. Suppose the expression you're looking at is x * y, but you don't know what x is. Then simply type x and press Enter. MATLAB will spit back out at you what it thinks x is.
4. See if the results of steps 2 and 3 are the same. If not, you may have a problem.