What is the best way to determine if an entity is an NPC? Is it

if (ent->NPC)
{
//is NPC
}
else
{
//isn't NPC
}

That works, so does...
if ( ent->s.eType == ET_NPC )
I seem to have used both in JMMod without any trouble. To be honest, I don't know why you would choose one over the other. Both are used in the SDK, maybe each is used by different programmers.

You might also want (for example)
if ( ent->client->NPC_class == CLASS_VEHICLE )

It was more of a curiosity question mostly to see how everyone else does it, thanks for the reply though. :D