Large bazaar-style open source projects usually don't just have one but many maintainers who review pull requests. Those often specialize in specific sub-modules of the application, so they have a better view of what is and isn't an appropriate contribution for that particular module.
There are a variety of ways to validate pull requests:
Some projects expect there to be an accepted issue before a PR is submitted. So, you can check the PR easily to see if it connects to an existing issue and then see if that issue is one that the team has said can move forward.
Hopefully on any large project there are a slew of tests integrated into ...