Some people just dip their finger into the warm liquid and if that’s the method you’re using, err on the side of warm bath water rather than hot because you don’t want to risk killing the yeast. In bread-making, I don’t like to guess and always use a thermometer.  For Red Star Platinum Yeast, the water should be warmed to about 120ºF to 130ºF, which is notably warmer than most other instant dry yeast, which typically call for temps in the 100ºF range. Follow the manufacturer’s recommendations for whatever yeast you use. I urge you to buy a food thermometer. It could save you from a bread fail and if you’re going to go to the work of making cinnamon rolls, having one is a no-brainer, not to mention you can use it to check meat or make candy.