If you are driving these spindles with conventional VFDs, they are almost certainly induction machines. While you can run PM machines sensorless without a position signal, you generally require an encoder if you are using a std VFD. That usually requires an expansion board, assuming your VFD supports it.

The slip frequency is easily deduced if you look at the motor plate. The rated power is usually specified with a rotor speed. With 50Hz, the unloaded speed for an IM would be 1500rpm (4 pole) or 3000rpm (2 pole) in the UK and 1800rpm or 3600rpm in the US. Typically you'll see ~1440rpm or so at rated load for a 1500rpm 50Hz machine, which is 96% of synchronous speed. For a PM, the speed will always be 1500rpm etc unless you exceed the breakover torque and it stops being a motor.

If you set the VFD display to show current, you will get an idea of the torque. However, there may be a torque signal available. I see from the manual for my Yaskawa V1000 that I could display the actual torque as a % of the motor rated torque. I've just changed mine to that setting, rather than show the spindle speed, which is already displayed at the controller. Some of the programs such as HSMA / FSWizard, Sandvik Machining Calculator etc give you an estimate of the required spindle power. I wish Fusion did this, even approximately, so I could dial up the load on the tool when doing the CAM to make best use of the motor. I have 3.3kW available apparently but it seems I rarely get close to using that when I have bothered to get an estimate of the spindle power required. I wonder how close to the limit people tend to push their machines?