I think strong human players find positions complicated where a lot of pieces are en prise, or one side's king is exposed, or there are a lot of possible captures.
Chess engines don't provide information on complexity directly. They do report "depth", i.e. how many moves into the future they are calculating each line before they evaluate it. In positions where many lines lead into positions with a lot of possible checks or captures, an engine will often take longer to move to the next depth as they have to extend each line beyond the reported depth in order to reach a quiet (quiescent) position. You can use the speed at which the depth goes up as a rough gauge to what positions an engine finds calculation-heavy.
It is worth noting that sometimes an engine will move through the depths quickly in positions that a human player would consider complicated, for example if the captures/checks peter out after a few moves or if there are tactics where quiet moves are key. Alternately, an engine may do a lot of calculation in positions where a GM sees a clear strategic goal that takes priority.