Depends on the job.
For some, manually. Every now and then, I look at the logs. Either that, or someone tells me something isn't working right, and I immediately know the scheduled job failed. Rarely happens, thankfully.
For others, I have emails that come to me daily that go into success or fail folders. For example, jobs that have to connect to other servers, such as via SFTP... I want to know if they fail, so I have the job email me either way (success or fail). I rarely get fails, but I'm sure glad when they come in.