The thrusters or the rams for pressing the metered amounts of product into the metering seats are equipped with force transducers which emit an electrical signal proportional to the thrust exerted during the pressing step. This signal is transmitted to the processor for controlling the operation of the packaging machine, which compares the signal with predefined reference values, in order to check whether or not the weight of the metered amounts lies within acceptable values. If it does not, the processor causes expulsion, from the unloading flow of the full capsules, of those capsules with metered amounts which have an unacceptable weight and which, if necessary, may be weighed using a precision device connected to the processor which may use this signal to confirm the operation of the transducers or correct it automatically.