Trick question, but easy answer. You will have to eliminate all other possible causes of distress (root issues, pests, etc.) before coming to the conclusion that the hoya is unhappy due to cold conditions. Many times, the symptoms of cold distress are similar to that of pests, rot, etc.

Cold damage looks really odd. The leaves will feel strange (hard to describe, but leathery and stiff?), they may yellow, and sometimes they will get an opaque look to them. The plant may physically droop and wilt, as well.

Rather than step in to help when symptoms start to show, it would just be best to bring the ones that suffer in cold temps, before it gets too cold. My rule of thumb is: all that can handle temps down to 50F at night, stay outside until temps drop to less than 50F. All that can handle temps down to 55F PM, stay outside until temps drop below 55F.

Here where I live the average Winter temps are about 36F to 55F (I had to google conversion because it's a celsius part of the world here :D ) Last year I brought most of them inside so I think I'll be doing that again this year. I'm more worried about some of the warmer loving ones. I can leave them in the plant room where it gets colder but has a big window with lots of Sun or bring them into the lounge room where it is warmer but not much light. Decisions, decisions...

I've been googling heat mats but I'm not sure what brands are better and whether I should get a thermostat as well.

A few of my hoyas got frost damage one night that it got under freezing temps during the night. The wind had blown the window open (swivel window. Hubby forgot to add a latch for cold windy days) anyway, leaf damage is all. All survived those few hours. This is freeze damage on leaves.

My plant room window doesn't open, thank goodness! I'm reorganising everything in there soon so nothing is too close to the window during Winter. Some are getting a bit too much sun at the moment. On the upside, my obovata looks nice with pink flecks now :D