This constitutes a distribution from the old 401(k), so the old 401(k) plan is required to issue a Form 1099-R. A Form 1099-R for a direct rollover to the new plan will have code G in box 7 and will have $0 in box 2a as the taxable amount. Unlike with IRAs, a funds in a 401(k) can only be moved by distribution and rollover, not by nonreportable trustee-to-trustee transfer.
You should make sure that the check is made payable to the new plan for your benefit and should indicate the 401(k) account that is to receive this rollover. Assuming it's from the traditional account in the old plan, it should indicate that your traditional account in the new 401(k) plan is to receive this rollover. With the check made out to your new 401(k) for your benefit, you'll simply forward the check to the new plan. You are simply involved in delivering the check, you are not involved in the financial transaction itself because the funds are not made payable to you.
