From there, all that you need to do would be to click on individual devices, where you can then check out whether the updating process is being worked on for said device. So far, T-Mobile has listed three levels for an update: Manufacturer Development, T-Mobile Testing and Completed. It is not a surprise at all to see how the only units completed happen to be the Nexus 5, Nexus 7 and the Nexus 9, whereas the Nexus 6 is being tested by T-Mobile at the moment, with the remaining devices being on the first tier at Manufacturer Development.
As for the T-Mobile devices which will soon obtain the Android 6.0 Marshmallow update, they include the HTC One (M8), HTC One M9, LG G3, LG G4, LG G Stylo, Samsung Galaxy Note 4, Samsung Galaxy Note 5, Samsung Galaxy Note Edge, Samsung Galaxy S5, Samsung Galaxy S6, Samsung Galaxy S6 edge, Samsung Galaxy S6 edge+ and the Samsung Galaxy Tab S2. [T-Mobile Support Page]