An ionic bond is a chemical bond between two atoms in which one atom seems to donate its electron to another atom.
So, to check if a bond between two elements is ionic or not we have to look for the electronegativities of them. Depending on the difference between the electronegativities of the two atoms the bond can be ionic or covalent.
Let's take NO
The electronegativity of N is 3.04
The electronegativity of O is 3