The only defense you have against fraud in bigger cities is to hold off results until they have posted results. The fraudsters like to know how many votes need to be manufactured.
We are at the polling place at 6 am to get things set up and ready to go by 7 am. We have to let anybody vote who is in lime by 8 pm. By the time the votes are counted, the machines are read and the the absentee votes are read in and tabulated, it can be 9 pm or even later in a heavy turnout election. Then, the judge (that's me in our precinct) drives the results and other materials into the courthouse where we may be another 30 minutes or so waiting in various lines to turn them in. I'm lucky to be home by 10:30 or 11 pm. Meanwhile, some precincts in Philly are still rotating people and taking votes, something those of us in the honest counties are not allowed to do.
The real solution would be to adopt an electoral vote award system as they have in Maine and Nebraska-- two for the statewide winner, one for the winner of each congressional district. That alone would considerably lower the potential reward for fraud-- 3-4 electoral votes, not 20.