thank you for feedback jarnoldbrown,
current draw sensing is surely feasible, each welding machine draws about 3 KW at full power at 240V...
as i said before i was thinking that an RF sensor could be cheaper... I remember that when i was i boy and i had a CB radio i built a very simple device, taken from an electronic magazine: it had a copper wire wound around the antenna cable and whenever the transmitter was on an LED would lit up... cant remember how it was done but something like that would suffice for my scope