Does a PID need to be "online" to be tested? Can the output signal be viewed without it being used?
The PID can be tested in an open-loop setup, however any integral gain &lt;> 0 will cause the PID output to run away, so you are likely to hit clamping (like control voltage maxima) with your test rig.

What really matters and needs to be tuned is the closed-loop behaviour. This includes your plants behaviour. Stability is prime goal, maximum overshoot and settle time (to set-point change or disturbance) can be tuned.

If you cannot tune in the live system, modeling and simulation is the way to go (look for Simulink Control Design). You would typically start with an empirical plant model, tune your PID for it, then run the actual device with this PID but a set-point well below Tmax. Instrumentation will give you data to improve your plant model. Repeat this development cycle a few times until you are confident to increase to the actual set-point.