Slipstream has a castbar. When you target Garuda (or her buddies), any spells or actions that charge up will appear over their health bar with a progress meter. It will literally say "Slipstream" and have a yellow bar sliding from 0% to 100%. When it hits 100% she will use it. SHe also will not turn or move during the casting, so this is when you move through her hitbox to behind her as slipstream is a cone in front of her that damage+stuns you.

Other moves like "Wicked Wheel" (circle AOE around her) do not have a cast bar and just happens at a set time. These you have to leanr the timing of yourself as all fights are tightly scripted.