Enforcing laws against the homeless is not the done thing in Seattle. The problem is not thinking of ways to detect their law breaking, but rather that the city chooses compassion over order and consequently ignores their lawlessness instead of enforcing the law.

If you do come to Seattle, you'll find the streets teeming with the homeless who are completely undisturbed by the police. Just walking around the city, and trying to avoid the homeless, I've witnessed multiple instances of homeless shoplifting, hard drug use (injecting themselves, crack pipe, etc), bodily functions on the street, threatening behavior (homeless people simply screaming invective and threats at passersby), to say nothing of people camped in sleeping bags on sidewalks and tents.

You can absolutely get on buses without paying. Simply ignoring the driver and walking by is all that's required. The police will not be notified, nor would they respond if they were, nor would their response be effective deterrence if they did.