A fake hug can be a deceptive indicator of affection. From a distance or up close, involved or not, you can make a quick easy read as to the level of affection (at the given moment) of the two people involved in a hug.
Affectionate hugs have the following qualities: Closed eyes, palm (not fingertip) contact and hips close.
If none of these are present, the hug is truly insincere. If any of these cues are missing and some are present, the hug is quasi sincere. Also, subconsciously, patting the back means release the hug.
Pay attention to hugs and look for these cues. If someone is professing affection for you or someone else but are giving insincere hugs, they are being deceptive. Also, if someone hugs your spouse or significant other in this manner and they are not a close friend or family, get a good lawyer.