My #1 way is from impatiently touching the board to see if it's cool enough to touch yet. That sounds dumb and it is. More generally, with iron soldering only

My #1 way is from impatiently touching the board to see if it's cool enough to touch yet. That sounds dumb and it is. More generally, with iron soldering only the iron and the last couple joints are hot. For SMD, there's more places for the heat to go; sometimes the entire board can be hot. Sometimes, you might need to balance being close enough in to get a good grip on the tiny parts, but far enough to not get burnt. You will feel the heat when SMD soldering - it's not always dangerous but another thing to pay attention to.