Designating Retirement Funds as Pre vs Post Tax

I have some IRA accounts (pre tax and post tax - Roth IRA). I also have a 401(k) with my current employer that has part of it as pre-tax and the other part as Roth 401(k). However, the 401(k) is all under one account. How do I segregate the Roth and non-Roth funds so I know how much of my retirement is post tax? I tried just starting a new account designated as a Roth 401(k), but there was no where that I could see that that could be designated.