Where can one find a aircraft thrust rating using the aircraft tail number or Boeing serial number.
Just to add extra explanation: it's possible to locate exact engine version. However, there are models where a single type (with an exchange of EEC) can change its thrust rating between a number of preselects. CFM56 is an example, 3C1 version I looked at could change the rating between 18.5k lbs and 23.5k lbs (with two other options at 20k and 22k).
So basically, you'll know engine version. But for some versions, it's not possible to learn the thrust, except at carrier ops/maintenance. And actually it could change any other day, should the carrier decide it's what they need.