Now, it's very common for bad guys to fake injuries when being arrested. Barring an obvious situation that warrants the ER they're probably going to take them to the jail and examine them there.
If the police are (demonstrably) unable to detect the difference between fake injury and real injury, what do you suppose the procedure ought to be?
I mean, really. This disturbing excuse of, "well, a lot of guys pretend to be hurt, so we couldn't detect this (umpteenth) one dying," should be a wake-up call that police should stop pretending to be trauma ward triage. Are they really okay with people dying in their custody? Are they really okay with this and think they should just keep letting people DIE in custody without a trial? What kind of monsters do that?
It's not reasonable to expect the police to be able to tell--that's an issue for a doctor, not a cop.