Checking for the presence of a DPF where it should be fitted is part of the MoT test, so unfortunately it can make it difficult for buyers to check it is in place given that some owners are trying to get around the law. Commonly the DPF is cut open, the internal filters removed and the casing welded back up, so fresher-looking welds on the outside can be a tell-tale sign. You could also have the vehicle inspected by a trusted garage or companies such as the AA that offer complete vehicle inspections for prospective buyers and ask them to check for the presence of a DPF.