How can I tell if I have traditional 401k or Roth 401k?

[LindaA]

If you contributed to your 401(k) plan, look at Box 12 on your W-2. A traditional 401(k) will have code D in Box 12, while a Roth 401(k) will have code AA.

What do all the letter codes in Box 12 of my W-2 mean?  https://ttlc.intuit.com/replies/3300234