Assuming you have a calibrated generator, and an o'scope which has a perfectly flat response within its specififed bandwidth (or a chart/formula to show/calculate the amount of response change with frequency), all you need to do is inject equal-amplitude signals at regular frequency intervals and meaure the input vs output of the filter, and record your data. You will then be able to build a graph of the filter response. Better yet, use one of those circuit simulator packages such as Circuitmaker or Electronics Workbench. Electronics Workbench even has a filter design wizard which really speeds up design.