i usually bring 3 plastic bags with me and whenever i see people not cleaning up their dogs' popo, i will give them my extra plastic bag and ask them to pick it up.
i also see neighbors fixing warning signs on their lawns reminding people to pick up their dogs' leftover.
To catch off-leashing in unauthorized area is easier, cuz very likely they'll stay there for a certain period of time, good enough for the city to come in time before they leave.
However, at the time you call the city about someone didn't clean up after dog, they'll be gone in 60 seconds! Unless the city sends some staffs hiding at the reported area(s) and wait for those irresponsible dog owners la.