the only way to know if your ductwork is OK is to measure it's dimensions, lay out all the fittings on paper, and evaluate the layout for CFM, Distribution and air Speed. also, the ductwork needs to be in the correct position, and the proper registers for throw and distribution in the rooms. was a blower door test done on the ductwork for leakage? if not, then it's doubtful the ductwork is "OK"...
in the 80's it was rare to seal any fittings to the plenum, and rare to seal the take offs to the registers.
here's a test for you.
pull any register off the wall, and look into the duct. is there any sealant on the ductwork? if not your ductwork is inadequate for your house by today's standards of energy efficiency, unless ALL your ductwork is inside the house. nothing in the attic, nothing in the crawlspace.
The contractors (the two I'm considering) have been in my attic. The others didn't go up there and weren't invited back after they tried to "sell" me anyway. The two contractors have seen the sizing and layout of the ducts and verified the positioning of the registers. The FPL energy survey included a blower door test. Ducts had already been sealed at the plenum (current A/C is not original 80s equipment and neither is the ductwork). The only leaks detected were where the ducts meet the boots at the registers. There is sealant in the ductwork at the registers now because I stood there and watched while those repairs were made last week.
What else can I do to be sure my ductwork is adequate?