From another thread:
There have been several occasions where I’ve shot someone on my own team because their name didn’t appear (and I usually wait for several seconds before opening fire “just to make sure”).
I’ve also seen the reverse situation, where I’ve been looking at someone up close (usually from behind) who I suspect to be a Covert Ops in disguise, but since the nametag appears I’ve assumed I was mistaken, only to be taken out shortly afterwards by the same character 
The manual states:
Players can still identify disguised enemy Covert Ops. Disguised enemy Covert Ops at longer ranges will display the name and rank information of the player whose uniform they’ve stolen but at close range, disguised enemy Covert Ops will not display ANY name or rank information and may be safely shot.
Looks like this isn’t 100% reliable 
