How can you tell if an equation is dimensionally correct?

You do the calculations, not with actual numbers, but with dimensions. For example, [length] + [length] = [length], [length] x [length] = [length]2, square root of [time]4 = [time]2, etc. In the final result, left and right of the equation have to be equal. For more information, search about dimensional analysis (for example, the Wikipedia article on dimensional analysis might help).