Once you have several informants you can use them on rotation to allow time for their suspicion levels to recover. If you can get some max-sec guys with long sentences they'll serve you for a very long time.
When you have the Informants menu open any inmate with an unknown reputation ( icon when you hover over them) will be highlighted in red - it's easiest to check for these during Eat or Sleep time. In addition to giving intel on contraband, informants will also reveal unknown reps within their coverage area. Activating an informant with 100% coverage will uncover all unknown reps across the jail, even if the informant is on permanent lockdown and the prisoner has just arrived!
I haven't seen an angel icon on PS4 - I think that must've been from the late May update on Xbox? The best way to find snitches (mouth+ear icon), ex-cops (broken badge icon) and ex-prison guards (ditto) is to check prisoners as they arrive off the bus - or as they leave in the hearse! If you activate a 100% CI just before 8am then any hidden reps in the new intake will be uncovered.
That's too easy for me though. I prefer to send them all to permanent lockdown in the Protective Custody wing with a bugged phone in each cell. By the following day any guys with family members will phone home and reveal their rep info. Then I keep the vulnerable ones in PC, move the most dangerous ones to SuperMax lockdown and put the others into med/max-sec.
I've just noticed that some (but not all) snitches on PS4 have a skull & crossbones icon over them in the Informants view. I assume this indicates the inmates that have been marked for assassination.