Preamble: I am a graduate student at a large program, about to start a postdoc, so I can speak about what has worked for me at the (hopefully) beginning of my career.
One thing I do occasionally is to check the upcoming conferences at the NSF Mathematical Science Institutes. These institutes tend to run many strong conferences. Since I'm in mathematical physics, I also try to monitor the upcoming conferences at the Simons Center for Geometry and Physics and the Centre for the Quantum Geometry of Moduli Spaces, and I'm sure there are centers that I'm not aware of.
The second way I find out about workshops and conferences is via announcements forwarded to our department's mailing lists. My adviser occasionally forwards announcements to me. But the most important way I find out about workshops and conferences is by talking about upcoming opportunities with my peers. Socializing at department teas is a good way to do this, and you can certainly directly ask people whose interests overlap with yours about their upcoming plans. Moreover, as you attend conferences, you should talk to the other participants about related upcoming activities.