Do you check voltage while the circuit is under load (such as a 1500 watt heater)?
Yes and no. My first test is just to see if the outlet seems OK. After we plug in I have voltmeters to monitor the voltage of all my incoming power - AC and DC. Then I can watch it under load. Can't really put it under load until we hook up. So I just check voltage and polarity and ground first.
An autoformer may be a better choice especially since they are easily available at Home Depot in Mexico.
I do realize that most people may not be able to acquire an ISB regulator or justify the cost. But this is what works for us. I do not know what an autoformer is. The ISB device can be ordered at any ferreteria and some even have them in stock. I know many homeowners and businesses have them as well as many RVers. They work. I would have to know more about what an autoformer is but I do not so cannot comment. I have had the ISB regulator for quite a few years and do know it works. Works for us. Works for many others.