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You are confusing the names, RJ45 is the plug commonly used for ethernet connections, RS485 is an electrical specification for data transmission, I've never come across RJ485.
In both cases it is possible to attach a receiver across the lines to monitor the traffic but difficult, if possible at all, to transmit into the wires. With RS485 you just attach a receiver IC (I use ADM488 and 489 devices or their Maxim equivalents) to convert the differential data back to single ended serial. With ethernet it is a bit more difficult because the lines are typically floating and terminated, you might need to use a hub as a signal splitter or make a high impedance differential receiver so you don't load the original signal too much.
Brian.