How to differentiate old yam from new yam without cutting open

If you've been a victim of yam scams you'd know better than to accept just any yam the smart vendors give to you.  
 The annoying thing is, most times the vendors know the main color of the yam even without cutting into it but will tell you it is "White yam" which means the yam is good but when you eventually get home and slice into it you'd see the color change "chameleon style". I've been a victim severally in this type of scenario and it is not a nice experience at all especially when the yam is rejected after cooking due to its color. 

Not so good old yam

So lets discuss, how do you differentiate between wateryam, old yam and new yam or good yam and not so good yam. Also, what makes yam change color like the one above. 

UPDATE:::

Thanks to Madam Iquo Ukoh Of Iqfoodplatter.com

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