Roaming without a roaming charge usually doesn't present an indication it's occurring. In the distant past, the carrier tag would change, but it's now all hidden. (I remember driving in Kansas with my Voicestream phone, and seeing some odd local carrier names pop up over a decade ago).

The only way on my recent road trip I knew I was on a roaming network was when my friends prepaid AT&T phone would switch to "Emergency only" mode. Had his phone not been around, I'd have no indication anything was different.

Make sure to check the coverage maps carefully when considering prepaid vs other options. Had Rob not issues that same warning here before, I'd have been completely unaware there was such a drastic difference.