The IRA basis is the total aggregate basis over all Traditional IRA accounts. You can have as many IRA accounts as you want but the IRS treats them as a single IRA. The IRA custodian has no way of knowing how much basis is in any other IRA account held by a different custodian so the custodian cannot indicate the taxable part.
A 8606 for should have been part of the tax return that reported the 401(k) rollover to the IRA. Perhaps 401(k) statements from that time will show the before-tax and after-tax amounts. Bank statements form the time of the rollover might also show it.
When I rolled my 401(k) to an IRA there were two transactions and two 1099-R's, one for the before-tax part and one for the after-tax part. Check your records and your tax return from the year that you rolled it over.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
