To my knowledge, Macs this old do not have temperature sensors.
The closest you can get on, as far as I know, anything before 1998 at all, will be to install an external temperature probe of some sort and read it with a dedicated device. I don't know what type of probe would be best, but something with a somewhat long cord may be best, since the best measurement of running temperature on the CPU will come from the machine being closed up and set up how you'll normally use it.