Anhidrosis can come in varying degrees from severe to mile and can be permanent or temporary. You'll hear loads of ways to treat it, but there are not cures and even the treatments are hit and miss (i.e. seemed to work for one horse, but did nothing for another) although you find people that swear by many of them (I know an OTTB that I told to never sweat much -not that he really understands- because the owner keeps giving him Guinness to treat it and he's really gotten use to it
I've had to deal with it, but fortunately it's always been a somewhat temporary condition with my horses. Always had them improving some after the first year. Sometimes it's over in a year and sometimes they came off of it over several years or sweating progressively more.
It can range from not sweat at all to varying degrees of sweat that's lower than normal (a horse can also have hyperhidrosis which is sweating excessively).
The best first check I'd recommend is your horses temperature. If the body temperature is normal then I wouldn't be overly concerned. Any time it's warm enough that a horse should sweat the lack of sweat is a good first indicator of whether the horse may have an issue. The body temperature is a good follow up to let you know if the horse is dealing with the heat or not. With most horses that have anhidrosis the lack of sweat results in their temperature being higher than normal at times when they should be sweating to reduce their internal heat.
People can suffer from the same condition, but we generally have access to AC so it's easier to deal with.
It's not fun and if the temperatures get high it can be life threatening. I've only encountered it with horses when they've been moved to hotter, humid climate, but that's not a requirement. It can happen pretty much anywhere. It's just very noticeable in the very hot and humid climates and probably poses a greater risk from it because of the higher heat.