Only one term can be negative of the binomial

x^2 - y^2

-x^2 + y^2 which would be rewritten as y^2 - x^2

the powers on all variables must be even

x^2

x^4= (x^2)^2

x^6=( x^3)^2

etc

the number coefficients must be perfect squares

1,4,9,16,25,36,49,...

or any fraction combination of these: 1/16 or 49/81 for example. Also decimals: 0.04, 0.25, ...

You can always check on a calculator to see if the square root of the number gives you a "perfect answer"