Determining Effect of Failure without a DFMEA (Design FMEA)

I just started working with process FMEAs at an organization that does not design products. How do you determine the effect of a failure and the severity without your customer's design FMEA or other input? We can SWAG but not real comfortable with that approach.

Did you try talking to your customer to get their input? in the absence of that you can use an industry RPN guide such as automotive's. When I was doing allot review of supplier's FMEA's almost no suppliers used our DFMEA's as input to the PFMEA. Ideally they should but that link never occurred. As an SQE I would review and challenge their RPN's.