If a sale is being handled by a realtor, in my experience usually the realtor will contact the road association and ask how much the assessment is, if the road association fees have been paid, and when they are due. The realtor wants to make sure there are no outstanding claims against the property being sold.
If it's not being handled by a realtor, and/or you find that a sale has gone through without the assessment being paid, I think I would first get in touch with the buyer and give them a friendly welcome to the neighborhood. Then I would let them know that regrettably the seller was not up to date on road association payments; and that in order to have a clear title they need to pay the assessment, but that they should seek reimbursement from the seller. I did have this happen once, and they buyer paid the assessment and said they weren't going to bother pursuing the seller for it.
This is one reason why it's important to stay on top of the situation, especially if you have a chronic non-payer. It's not so bad if the association loses one year's assessment on one property because a sale slipped through without payment, but if someone owes for several years, you really want to have current Notices of Claim on file at the Registry of Deeds. Still, if a sale does slip through, it is my understanding that you can still file in Small Claims Court even if there is no Notice of Claim on file - although I'm sure you'd have a stronger case if the Notices of Claim have been kept current.