Hello, there are two blowers on that furnace, the main blower that is grey and 14" ciube size, and the much smaller inducer fan, generally black and with a small 4" diameter motor attached.
I need to know very specifically which blower you are referring to.
How long has the NEST been installed?
If the main blower comes on before the furnace heats up, and if there is now blower control on the wiring diagram, then the blower is controlled by a timer built into the printed circuit board.
The fan relay is most likely ON the printed circuit board, its small about 1/2" tall and 1" x 1/2" two wires from the main blower motor will attach directly the printed circuit board in that case.
We can do a heat stress test on the printed circuit board. Use a hair drier to slowly and gently warm the PCB to 125F... if it is defective due to internal micro cracks, those will expand and make the problem worse, or make the problem dissapear all together.
In either case the PCB is bad and needs to be replaced
We can go from there as needed.