We need to get this worked up into the Wiring API, but you can get a return value from the cellular HAL. You may not always be able to use the following code, but it should work right now (as of 0.7.0 & 0.8.0-rc.3). Note: these will also block with SYSTEM_THREAD(ENABLED), which might be a good thing
if (cellular_on(NULL) != 0) {
Log.error("Could not turn cellular modem on!");
}
if (cellular_off(NULL) != 0) {
Log.error("Could not turn cellular modem off!");
}