I use to design linear voice coil motors back in the day, along with FEA simulations and high temp testing. You can verify if your rotor magnetic strength is degrading if your KV starts rolling off or get a gauss meter, but as someone said earlier, you have to check it at the beginning to know. The Curie temp for the low grade neodymium magnets is around 310C. We didn't see the flux fields start decaying until about 110C. Your rotor is typically 20-25C cooler than the stator windings due to the airgap and thermal isolation. The biggest issue with running your motor hot is the derated electrical performance. These are the design rules of thumb we still use in our systems and there's a ton of military hardware flying around based on these rules. Just throw it on on the motorlyzer and see if the KV's seem reasonable. There's some guys who test motors and publish the results on youtube for reference.