Is al2o3 a ionic or covalent compound?

An easy way of determining bonding types is to look at the elements in the substance.

2 metals is most likely a metallic bond. The 2 or more metals are surrounded by a sea of electrons holding them together.

2 non-metals is usually a covalent bond in which each atom donates an electron(s) which is then shared between the atoms.

A metal and a non-metal is usually an ionic bond. The metal atoms wish to lose their outer shell electrons (valence electrons) and the non-metal atoms wish to gain valence electrons. The metal atoms donate electrons to the non-metal atoms forming cations (positively charged particles) and anions (negatively charged particles). The charged particles are then attracted to each other and form a 3D fixed structure. Al2O3 is an ionic bond and forms a fixed lattice.