If a seller is behind on monthly fee payments and special assessment payments, I'd be concerned that he/she is also behind on mortgage or tax payments as well. This will show up when your bank orders a title search. Unfortunately, banks often wait until a week or two before closing to run the title search.
If the association has sued the seller for past due fees, it'll show up as a lien on the property in the land records of the county. You or your agent can go to the county's Registry of Deeds web site to find this information. A seller's agent should disclose outstanding liens, but don't count on it.
In Mass, condo associations who sue owners for unpaid fees get "super lien" status, meaning that when the property sells, the association gets paid up to 6 months worth of fees ahead of the seller's bank loan. However, the association is out of luck when it comes to special assessments, and they might try to get you to pay it.
Sounds like your agent and your attorney have some work to do before you're clear to close.
In addition, if the buyer is obtaining financing, the bank will require a condominium questionnaire to be completed by the Trustee or manager of the association. That questionnaire usually inquires about special assessments, condo fees, how many unit owners are in arrears, etc.
Provided the buyer is being represented by an attorney, or better yet, they are obtaining a mortgage, the Buyer will make sure that all past due monies are paid from the proceeds of the sale at the closing. Often times, the best thing that can happen is if a problematic unit owner sells - the situation is resolved.
If it is at all questionable as to whether the association will be paid, the association should retain an attorney - the unit owner in arrears would be responsible for attorney costs as well. Condominium law in Mass is very strong and in the favor of the association.