I use my fingers lol, no kidding. I rest the backside of my fingers on top of the soil to get a feel for the temp. If it feels slightly cool to warm that’s good but if the soil is kinda chilled you can bet the soil is even cooler down at the bottom where much of the roots are spreading out. I use heat mats under my containers when the soil feels a bit on the cool side. For me they are a must have for cool and cold weather growing indoors. I use them for seedlings year round too. Can’t remember the ideal root zone temp off the top of my head but I think it’s the range of upper 60’s to lower 70’s Fahrenheit.
I should point out though that I maintain the climates of both my tents by feel so I don’t have the usual climate control meters such as temp/humidity so If my gardens feel warm or cool to me then I adjust them accordingly. Plants and humans prefer very similar climates so if it’s comfy for me then it is for them as well. The only tools I have are a ph meter, an ec meter and measuring utensils.