If the resin is in powder form then used high power magnets to remove them, and if it is in the form of granules then melt it in big volume in any bucket type (in this way metal parts will moved down in bottom) then in its molten state put it in extruder for re-granules or processing (molding).
This is only one of many potential issues when using reprocessed resin. There is virtually no way to verify or certify the origin or content of the reprocessed material other than an IR scan on a batch by batch or lot by lot basis. Then, you still need a baseline to compare the scan results to. Additionally, you are exposed to the possibility of small amounts of contaminants throughout any given batch or lot of material that a scan on a small sample of the batch could very well miss completely.
Depending on the end use, I would refrain from the use of reprocessed resin of unknown origin on nearly any and all applications. Only the most basic products and non process variation sensitive, non color variation sensitive–(black) applications should be considered. Anything that has any strict load or strength requirements should also be ruled out.