Usually airports use signs with both graphics and text for this. Removes the ambiguity unless you mistake the image of a bag for the tray and vice versa.

Which leaves me confused with GP's suggestion. Every TSA line I've been in already has these signs. Are airports in other countries worse about their signage than TSA?

There was zero signage at the last few American airports I flew through. Four airports in 2 weeks and they all had different rules, and felt the need to yell at you for not knowing their nuances.

There isn't a standard design for the signs, so each country designs their own. (If they exist.)