They use the normal frequencies for the Angeles which is 172.375 for Forest Net, used by fire and law enforcement units. The Forest's admin net is on 164.9375 and is used by everyone else. During multiple fires the admin net is sometimes used as a second command frequency for fires. They have the normal NIFC tactical and command net frequencies programmed into their radios. Take a look in the Wike portion of this website under "National Incident Radio Support Cache" if you don't know those frequencies. Also used and heard is the R5 Project net on 164.150 as a day to day activity simplex frequency for all functions (fire, LE, recreation, resources, etc.).
The Angeles has 14 repeaters and a handful of microwave linked remote bases. When the latter is used you can't hear the mobile end unless you are fairly close by or the unit is at a high elevation and not down in a canyon. Listen for the morning fire resource lineup on 172.375 about 0930-1000 and you will hear every station being queried for the days staffing. This is a good time to figure out what you can and cannot hear from your location.