Most field guides only mentions that this species is identified by the white bars on the breast (not spots like Red-billed Firefinch) and brown crown. These are good characters at close views.
Another good character that I often use in Ethiopia, especially at distance, is that Bar-breasted Firefinch is obviously darker than Red-billed Firefinch.
Gibe Gorge is one of the best places in Ethiopia to see this species. More than 50 birds in a day is not unusual here.