The absolute best proof that your plan is a qualified plan in the eyes of the IRS is the IRS determination (or IRS opinion) letter that we supplied with your Solo 401(k) plan. The letter is from the IRS and shows that the 401k meets the IRS guidelines for a qualified plan.
The plan’s qualified status is what allows you to receive your tax benefits including:
tax deductions on new non-Roth contributions to the plan
tax free distributions on Roth Solo 401k funds
tax free rollovers from existing retirement funds
You can find a copy of the IRS determination letter in your 401k plan documents.