oil shimmering or just smoking?

I only can provide an incomplete answer, because when double-checking what I thought I knew I discovered that there seem to exist in fact two opinions on this topic. One is that oil should never be heated to it´s smoke point when cooking, because there the ingredients of the oil start to disassemble to burnt products that are unhealthy to consume, while the other argues that the smoke point still does not mean that the oil is effectively burning up and only some minor fractions of it´s contents start do break up and evaporate. E.g. compare here vs. here.

I personally prefer to go on the safe side and use heat resistant oils or fats like sunflower, canola or clarified butter fat for browning and frying.

It doesn't matter. All you need is for the oil to be properly heated. You can use the cue which is most convenient for you - shimmering, smoke, an IR thermometer, smell, throwing stuff into the pan, or your spidey sense that suddenly reminds you of the pan after enough time has passed. Go with whichever is most convenient for you, they are all correct.

That being said, shimmering is usually visible in stainless steel pans, but more difficult to notice in pans with a dark bottom such as seasoned iron pans or PTFE coated ones, so it might not be the easiest one to pick.