Most of the time you can request id from almost every suspect and witness.
you can request for every witness ID and also sidewalk witness as well.
You can request for witness id even its not necessary. You can still get bonus event such as fake id user / having illegal weapon - refer search/ frisk

You can request a person after detaining them

You can request for ID when you detain vehicle if you
-Detaining at traffic stop
-Detaining due to vehicle violation

Lastly you may also ask id for those whom violate simple rules such as
-jaywalking
-littering

You may frisk anyone when they show suspicious behavior such as
- Looking around
- Green crumbs
- Fearful
- Shaking
- Angry ( Currently unsure as it is in handbook but seems to be not ACCURATE ) - Dev mistake either in handbook / Bug

All arrested suspect can be search which include

DUI
Illegal weapons / items possession
Running away from cops
Events - Assault / Robbery / Etc..

You may do a drug test for all the accident suspects and also vehicle detainment such as

- Accident suspects ( To increase report / get score if positive )
- Traffic stop
- Detainment for vehicle violation
- Traffic violation
*Underglow lights
*tinted windows ( BLACK )

Most of the time you dont have to do it if they don't show sign ( EXCEPT ACCIDENTS )
- Nervous
- Red eyes
- Smells alcohol / drug

You may also conduct vehicle search when they show sign of DUI
-Green crumb
-Drug smells
-Flee from detainment
-DUI
-Illegal Item Found