Any way to make sure whether the soda is diet?

Test them with your meter.
Or use those pee on glucose sticks.

Wow glucose sticks is a genius idea (they’re a lot cheaper than strips)

Order fizzy water instead?

Order it in the bottle/can, and have it opened at the table?
Also a problem at bars with the squirty dispensers.

Hi,

As someone who regularly drinks diet in venues due to being "designated driver" & part of the band with the live music scene.
The sugar stuff seems a little stickier to the touch if dripped on a wooden table? But that is pending on how clean the tables are in the first place...

Some of the "spit & sawdust" places can or used to do bottled from a fridge behind the bar? Which is easier to spot.