If it's relatively modern cable it might well have the size embossed periodically on the outer sheath - but you may well not be lucky enough to be able to see the appropriate part of the sheath! Failing that I'm afraid that it's the calipers or micrometer!Is there an easy way to determine 1.0 or 1.5mm² cable or is it a case of getting the vernier out?
Kind Regards, John