When you buy a condo, you can view the financial documents in advance. You should be able to see how much money is in reserve, and what each reserve account is earmarked for.
You will not know if a specific owner is not paying their fees. But if an owner falls behind, they can be assessed late fees. All unpaid fees become liens against their title. When they eventually sell, the new buyer will not get a clear title if the fees are not paid. So the title company or attorney will find the recorded liens, and hold that money back from the sellers proceeds, and pay it to the Condo Association. It's the only way a buyer can get a clear title.
The only way other unit owners would be responsible for another owners unpaid fees would be if there was not enough money in reserve. But you'll know that before you buy =)
Regardless, there should be money in reserves to cover any short-comings in the