How can I know this 16 Gb is supported in my T420? I-7 2620M processor

It works in both my T420 so I see no reason why it would not work in yours.

Questions & Answers: CMS 16GB (2X8GB) DDR3 10600 1333MHZ NON ECC SODIMM Memory Ram Upgrade Compatible with Lenovo® Thinkpad T420 4177, 4178, 4180, 4236 Series - A13

double click image to zoom in

CMS 16GB (2X8GB) DDR3 10600 1333MHZ NON ECC SODIMM Memory Ram Upgrade Compatible with Lenovo® Thinkpad T420 4177, 4178, 4180, 4236 Series - A13

Computer Memory Solutions guarantees compatibility with models provided in description.
Memory Type: SODIMM, 204-Pin
Speed: DDR3-10600, 1333 MHz
2 Piece Kit
Product appearance may vary. Picture for reference only. limited lifetime warranty when bought directly from computer memory solutions