If you mean leaks in the ducting, just look for the big sucking wounds.
Seriously, any significant leak in ducting would be obvious. Small leaks
that would require sniffing with smoke are meaningless in such a system.
Blast gates always leak too. Concentrate on eliminating leaks around the
garbage can lid, using weatherseal for example, and get rid of as much
corrugated hose as possible. Smooth ID hose or pipe makes a big difference.

I agree with starting with the big stuff, but small leaks can add up
quickly. On my system (2hp Oneida) plugging 5 small leaks (couldn't
see them, but smoke found em) increased cfm at the tablesaw from
510cfm to 680cfm (@ 7"h20). This made a noticable difference in the
amount of dust left on the saw.