J-Man does not have to see it, he can just spew fire everywhere and detonate every single nano bombs.
Of course you are asking, What the hell is J-man breathing fire everywhere for? Well, if Deidara does use C4, I can only see two options for J-man(Sennin Mode), First he can use his hair as a shield for the explosion but he might die due to inhalation of the nano bombs Or he can use fire to fight fire. if J-Man(Sennin Mode) has enough force to cause an equilibrium with the opposite force which is the C4, then he would be safe and as for the nano bombs going to J-man's direction, they would be destroyed too.
I just hope those Nano bombs are not Heat Seeking missiles haha.