Encrypted Tor packets stick out like sore thumbs (for one they are all padded to 512 bytes + headers). Technically an ISP could detect that you are using Tor by looking for streams of packets like this (and various other things). It is kind of an arms race. So far no government is known to be fingerprinting Tor traffic in this specific way, although some governments have located Tor users via various traffic fingerprinting attacks. It is harder for this sort of attack to be done than detecting Tor users via the IP addresses they connect to. It might not even be feasible to scan everyones traffic arbitrarily looking for some of the fingerprints associated with Tor traffic. In other cases it can be done though, as has been shown by various governments. When governments find a way to detect Tor traffic that they can actually implement large scale, Tor tries to take counter measures.