hi, I'm having problems recognizing what is a perfect square and what is not. Could anyone share so basic stuff i need to know about them , i'm very lost with them and I have no idea what to do with them. For exemple : $\displaystyle \frac{5x^4}{6}-\frac{3x^{-4}}{10}$

Can i get a full demonstration of why this is a perfect square and tip to recognize it thanks alot :D