For the A320 we can use the "green dot" speed (which is the single engine best lift/drag speed) to determine the gross wt. of the aircraft in flight. eg -
Green dot spd ......220 Kts
Altitude............31000 ft
From 220 subtract 11. (this is one knot for every thousand feet above fl 200) = 209.
From 209 subtract 85 (CFM), or 80 (IAE) so 209 minus 85 = 124 (CFM) then divide by 2 = 62. 62,000 kg.
This is in the FCOM.
Hope this helps.