The same way they always did. Stakeouts, eyewitnesses, and plain old-fashioned gumshoe work and boring waiting.
So then people can spy on those that they do not even know? How is that not a violation but you having an ID in your wallet isn't? Which is more instrusive?
You really aren't making any sense now. A stakeout has a target, a warrant has a target that target is a specific ID. You know, the thing you call unconstitutional.