You can't tell who is a diamond smurf based on CS alone. Honestly, in lower elo players fight far more often when there is nothing to fight for, so cs/min numbers tend to be lower. Like in Diamond its not uncommon for me to have between 8-10 cs/min but when I'm on a smurf I will likely have somewhere closer to 7cs/min just because of the nature of the game at lower elo.
The real way to tell is to look at someones damage output if they are a mid or adc, it will usually be the highest in the game pretty consistantly. Win rates will usually be somewhere around 70% or higher.
Source: D5(aka fancy bronze) smurf