Many appear the same. There is often a metal or synthetic tag with number on it. Some have it etched on a flat spot. Plus some have added data stamped at or near the end of the output shaft housing.

Just curious what model year Impala and which engine you are looking to match?

Wikipedia has a long thread for GM and CHEV transmissions. Hang on . . .

Here's that Impala wiki so you can choose your actual model year:

https://en.wikipedia.org/wiki/Chevrolet_Impala

Notice each section shows engine and transmission (example image below). Tap the transmission for the matching engine. That takes you where you want to be.

Notice transmission options. Use those numbers to fuel your on-line search criteria for IMAGES.

 

Wow, thanks so much! Very helpful

 

Welcome.

Curious. What model year are you after?
Are you building a classic 1969 hot rod?!

 

2012 and later limited all came with the 3.6 and the 6T70
2000-2011 had several different engines with the 4T65E

The 2004-2009 SS Models used the 4T65E-HD.
2004-2005 SS had the supercharged 3800.
2006-2009 SS had the LS4 V8.

 

GM made a bunch of improvements in 2012 but there are a couple gotchas with the 3.6 and 6T70 used in the 2012 up W Bodies and the higher trim Epsilon.

If the previous owner didn't keep up with the oil changes and use DEXOS oil with the proper oil filter the 3.6 timing chains & vvt actuators can go to pieces.

The 6T70 3-5-R Drum can have issues. This is accessible without removing the transmission from the vehicle. Sonnax sells a drum saver kit to their distributors. Sonnax 6T70 drum saver