Smell it. 🤷🏼♀️
Some diapers keep baby drier than others, so you may want to experiment. A hot baby will be hot all over, not just the diaper area.
I had the same concerns when my son was younger. He would be wet all up his back. It turns out he was leaking through his diapers at night, not sweating from heat. We were also using pampers but once we sized up, he stopped leaking through!
I don't know how to tell if it's the diaper or sweat but in case it's sweat you can try swaddles/wraps/sleepsacks in different warmth levels. I really like the ones that have a TOG rating so I know what room temperatures they are best for.