Well, Showdown! has 3 Zapdos sets for natdex, but only the defensive set runs Heat Wave. Probably about 1/3 of them run Heat Wave, but another way you can find out is by doing a bit of damage to the Zapdos, then using the Showdown damage Calcualtor to see if it is likely the Defensive set, or the Offensive set. The Defensive set is usually the one with Heat Wave. Link to the calculator:
https://calc.pokemonshowdown.com/
Installing the Showdex can help too. Just search it up, and install it. it adds a built in calculator to Showdown! that updates in real time. It has helped me win many battles.