The 1099-R will have a box checkmarked for the type of retirement account this was distributed from. And look at the issuer name on the 1099-R, to see if that is the same as the employer plan account manager.

Only your client can tell you if they participate in Roth 401(k), 401(k), both. Example: it is possible your taxpayer contributes post-tax deductions to Roth 401(k) and the employer puts the match into the 401(k), because they are not allowed to put that into the Roth 401(k).