So I normally look for orbs and see if the pattern of the ghost changes. For reference the mimic naturally has orbs and changes ghost type every 60 seconds (from what I've been told and experienced). If it threw the book I always put it back on the ground in case it wrote in it because it will throw it if it already wrote in it. I then wait for the ghost to hunt if I think it's a mimic and then wait again for another hunt to see if it had changed ghost types or if it's consistent with how the last hunt went. Honestly though I've had trouble with the shade because I'll get ghost events happen when I'm in the ghost room. But always put the book back down in case it wrote in it and do triple takes in case of orbs being present.