The other devices can connect to obniz using BLE and read or write the value of characteristics. You can notice read/write by setting a function called when such an event occurs.
By specifying a function in peripheral.onreadfromremote, you can know that the value of the characteristics has been read. This function is called every time the value is read.
You can detect writing by specifying a function in characteristic.onwritefromremote. "address" stores the address of the central and "newvalue" has the value written by it.