Methods for figuring out the best schools for a particular field of study
Research groups that are active in a particular field will publish often in those fields.

So, identify conferences* in your field of interest. Look at the technical programs for the last couple of years, and check out the groups that seem to be contributing a lot.

Also, ask professors at your undergrad university who work in related fields.

* Also workshops, large conferences with a few on-topic sessions, journals, etc.