// If you have holes, you can do this
struct entity_ref
{
    u32 index;
    u32 generation;
};

bool is_entity_valid(entity_ref ref)
{
    if(entities[ref.index].generation == ref.generation)
    {
        return true;
    }
    return false;
}


// If you don't, you have to loop over all of them and check id