Put some little skulls in the cloud?

Additional stink lines around perhaps... And crossbones or toxic symbols around it lol

The face of whoever is exposed to it usually say it all. double so if theyr fur spike from it(if not fully fall down), if they steam afterward or have their face turn green or their eyes spiraling...if the gas is ovviously 'deviated' on the side from the prey face presence it helps too

Another classic trick is to ahve it happen nearby plants and have those wilters, or nearby flying bird suddenly fall down!