how can i know the dc motor torque

Try the manufacturer's data sheet

It's easy enough to measure it's torque at zero rpm (stall torque?) by mounting a bar to the output shaft and hanging weights on it, then slowly applying power.
The relative safety of this depends on the size of the motor (I deal in small 12v/24v Dc motors which draw about 1/2 and amp stall current)
Del

i can know the dc motor torque by measuring it with a torque meter .

Power is torque times angular velocity, therefore, if the nameplate gives motor horsepower, the torque (at peak HP) should be HP/angular velocity. There's a scale factor involved for a given unit of angular velocity, so look it up on the internet.