The 401(k) plan, in this case Fidelity, is required to track that for you. Still, 401(k) plans sometimes lose that information, particularly if there are rollovers from one 401(k) to another employer's 401(k) involved, so you'll also want to track that in your own records in case the 401(k) plan ever issues a Form 1099-R that shows a taxable amount that does not correctly reflect your basis.
Fidelity kept track for me on all my after-tax 401k contributions. You should be able to get a yearly before/after-tax contribution listing in your Netbenefits account
If your workplace offers a ROTH 401k, you'd be better off contributing any after-tax amounts in that vehicle.
Mine were before the ROTH 401k came out, so I never had the option.
After tax basis isn't used until you actually remove $$ from the 401k...either as a distribution, or sometimes as a rollover to an IRA or other company's 401k...But rollovers can get messy when they are a mix of before and after tax.
The 401(k) plan, in this case Fidelity, is required to track that for you. Still, 401(k) plans sometimes lose that information, particularly if there are rollovers from one 401(k) to another employer's 401(k) involved, so you'll also want to track that in your own records in case the 401(k) plan ever issues a Form 1099-R that shows a taxable amount that does not correctly reflect your basis.