if the compound is made of just two elements, if one is a metal (ie belongs to any of groups 1, 2 or 3) and the other element a non metal, (ie belongs to group 5, 6 or 7) then the compound is most likely to be an ionic compound. For example NaCl, MgO
If the compound is made of identical non metalic elements as in O2, Cl2 then the compound is covalent
If the compoud is made of just two elements that are both non metals such as in SO2, CO, NO, CCl4, the compound is covalent
If the compound is made up of more than two elements, such as in HNO3, Na2CO3, CuSO4.5H2O, you may need to break the compound into dissociating parts. You will see that, the compounds are ionic.
Hydrocarbons, compounds containing only hydrogen and carbon of varying molecular size are all covalent. Examples are C2H6, C2H4, C2H2
Note that there could be some little exceptions to the examples given. Mostly with first members of every group because of their small size which make them show substantial deviations from group behavior. For example HCl is covalent not ionic.
