How can you tell if a compound is held together of a covalent or ionic bond by the chemical formula?

The formula of an ionic bond containing compound will always begin with a metal, which forms the cation. The rest of it will be a non-metal or a covalently linked compond of a metal or a non-metal with oxygen and, sometimes, hydrogen. Example: MgHPO4 is ionic: Mg(2+) HPO4(2-).

Covalent species are generally the non-metallic elementary or compound substances. Pure metals have metallic bond between atoms in solid, and, I think, also liquid, state.

Nitrate and nitrite and so on: you'll have to learn it by heart, there is no strict correlation with the number of oxygens. I mean, yes, there is some: -ide has none, then hypo-...-ite, -ite, -ate, per-...-ate and hyper-...-ate increase the number of oxygen atoms by this order, but the exact numbers vary.