How to know that a bond is ionic or covelent by just looking at it ?

Ionic bonds are made up of a metal and a nonmetal elements while covalent bonds are are made up of two metal. So, you can just look at the periodic table and identify which compounds have two metals and which have one metal and one nonmetal. Also, those compounds with high electronegativity difference are very likely to be ionic while the opposite is covalent. HOWEVER, some compounds can be both covalent and ionic. For instance, if HCI gas is at higher temperatures, then it is ionic while it would be covalent at room temp. 

Hope I've helped. 

            - Dotz