At some, mostly local-style, vegan restaurants you will see (small) yellow with red flags. It shows that their food is strictly vegetarian (nowadays called vegan).
There are Chinese-written flags but these also say ‘good veggie, good life’ in English so those are easy to spot. The Thai flags have the Thai and Chinese “tjay/jey” on it but no English so try to memorize the Thai (or Chinese) symbols for it! I try to remember it as a mirrored GI. Although it could differ a bit depending on the font.
I haven’t done it myself but if you want, you can print or draw out these symbols/characters to show when ordering vegan food. Check the Thailand Vegan Basics on how to order and use the word ‘tjay/jey’.
See pictures down below with all the variations.