Calculate? That's something on the manufacturer's datasheet for the motor I'm afraid - easier to measure it
with a peak detecting current meter really.
The fire risk is not from the relay, they fail in safe ways and are usually reliable if not mistreated.
Induction motors can overheat if stalled, or run at too low a voltage leading to stalling. Stall current
can be loads higher than running current, though it all depends on the exact motor design and
the construction of the rotor.
Good motors have thermal-cutout fuses embedded in the windings which melt if the temperature
gets too high and breaks the circuit - if you have such a motor its pretty safe... Others are designed
to take continuous stall (normally called a "torque motor"). Others are not, however.