Buy from someone credible who has a name/reputation to keep in this business. They will more likely give you a true het if they say it's a het since their reputation is on the line. Besides, it's not worth tripping over pennies on their way to dollars by defrauding someone out of a few bucks by selling them a normal advertised as a het.
Be cautious of "I bought this het from XYZ" type of guys. I'm not saying they're out to defraud you, I'm just saying "be cautious". When buying 66% hets, etc., ask to see photos of the clutch. There should be a homozygous animal in the bunch, along with some hets/possible hets if the breeding parents were het themselves.
One last word of caution. Don't buy ANYTHING from anyone selling het for spider! 
Ron