on a gel you see the size, something coming up with exactly the right size even at 35 cycles indicates that there was at least some of what you designed the pri

on a gel you see the size, something coming up with exactly the right size even at 35 cycles indicates that there was at least some of what you designed the primers on in the sample. if you want higher certainty, with three primers you can get two different sizes ruling out any doubt. with the rtpcr machine readout which only gives you the signal corresponding to how much was amplified over time you need tricks like the dual labelling.