Reversible vs irreversible reactions

When do you draw a reaction with an arrow in only one direction (irreversible) vs an equilibrium reaction arrow (reversible)? It depends on the type of reaction and the species involved. Nucleophilic attack is reversible when the nucleophile is a good leaving group. Proton transfer is reversible when the pKa's are within 6 units of each other. Loss of a leaving group is always reversible because all leaving groups are nucleophiles. Carbocation rearrangement is never reversible.