Can I just T into a CAN bus network if I just want to observe the data?
Yes, with some restrictions.  The typical differential twisted pair CAN implementation tries to keep the impedance of the transmission line to 120 &Omega;.  Anything that deviates from that, like a T connection, will cause some reflection of signals.

In practise, keep the T short and it should be fine.  The Transceiver chip, like the common Microchip MCP2551, should be only at most a few inches from the line you are tapping into.  The higher the CAN bitrate, the more keeping the Ts short matters.