From allrecipes.com:

Prick the potatoes with a fork several times, then rub potatoes with olive oil, sprinkle with salt, and wrap tightly in foil. Place the potatoes into a slow cooker, cover, and cook on High for 4 1/2 to 5 hours, or on Low for 7 1/2 to 8 hours until tender.

From the USDA Food Safety and Slow Cookers:

https://www.fsis.usda.gov/shared/PDF/Slow_Cookers_and_Food_Safety.pdf

Personally, I would never trade the difference in quality and taste of baking a potato in an oven for one hour versus 5-8 hours in a slow cooker. Russet potatoes are about 80% water and wrapped up in foil left that long in a slow cooker is just going to steam the potatoes. A possible solution, if just 2-3 potatoes, would be to turn on the oven to preheat and microwave each potato for 4-5 minutes, then transfer to the oven to finish off. At the low temp and sealed in foil I’d also be concerned about maintaining a minimum temp of 140 degrees F.

The only way to measure if the potatoes are warm enough in cooking in a slow cooker is to use a temperature probe.