Although there are affordable solutions to heat an enclosure, cooling one is costly and involves usually running an AC in the enclosure room. Cooling fans, as you have used, do help to bring temps down a few degrees. In enclosures with water can also do water changes with colder water, or even use ice packs in emergencies too. This is a subject that will be affecting many of us as the Earth heats up in the next couple decades .

In regards to your question on frog overheating signs, don't know of any that might work as a warning. Being cold-blooded animals, think by the time we see any acute physiological signs, serious damage would be done already. My idea would be to prevent temps going above the recommended maximun and if they do; to do any emergency type cooling available regardless of overheating symptoms in frogs .