Fortunately, the fakes are few and far between. These are covered, in sufficient detail, in the relevant listings. Copies are also somewhat small in number, but they are not made to fool anyone. Other copies are those tools that were made directly from an original tool, usually by a patternmaker for his own use. These are rather easy to spot since they are smaller than the originals and often have a grainier texture to their non-machined surfaces.
Repairs are the real bane of collecting.
It's been going on for decades in other fields, and is now part of the tool scene. If it's easy to repair, and to fool collectors of, mechanical banks, then it's a relative picnic to do the same with tools. Chips and missing chunks of castings are very easy to fill using finesse welding. The welds are colored to match their surrounding and then are machined to be unnoticeable. These are very difficult to detect, and one should always be suspicious of repairs to areas where the tool is susceptible to breakage.