I have an old house and am not sure if the walls are square. How do I check for this?

Use the 3-4-5 method. Measure along one wall for three feet (or a multiple of three—the bigger the better). Mark this point. Next, measure along the other wall from the same corner four feet (or the same multiple) and mark the spot. Then measure diagonally from mark to mark. If the diagonal is five feet (or the same multiple), the corner is square. If not, it is out of square.