How to differ live rock from base rock

Hello, so how can you see the difference between live rock and base rock with the eye...I am planing to buy some rocks from a guy and don't want to get ripped off...they are in saltwater in bins with water circulating

Base rock is bleach white to the lightest of tan in colour. Looks just like live rock in terms of shape and pourous appearance. Very clean appearance (no debris present). Usually sold dry. No good reason to sell it wet other than to extract money $ from your pocket.
Live rock will usually have some sort of growth on it (coraline, green or brown algaes, etc) but not always (ie if its kept in bins of salt water in the dark). Ranges in colour from light tan to tan/brown for "uncured liverock" to nearly completely purple for full coraline encrusted rock. There may be small clam shell stuck in the rock openings. You may see pods or featherdusters on rock that has been curing for a while.

Awesome info Matt...Couldnt have worded it any better!!..

Thanks guys!