Which begs the question: how can you tell?
For starters, Bob always signed his work. Prior to the famous "Chop" logo, he would stamp the letters LUM on the ricasso of the blade. See the picture of the two ivory tantos by Jim Cooper for an example. There are exceptions. I have one knife he has authenticated to be his where he signed the knife with his Chinese Lum character on the brass pin, but this was one of his earliest work.
Then there is his hand-rubbed satin finish. One of his signatures. It's very easy to spot the Lum finish once you've seen it before.
His grind lines. As mentioned before, Bob is considered by many knifemakers to be one of the best grinders in the business. His grind lines are always clean and symmetrical. The best way to describe it is "crispness", the lines are well defined and lacks a sense of "softness". Some of his grinds are easy to spot; the diamond tip on his daggers in an earlier post; the Hamagiri (appleseed) grind of his tanto tips. Others are not as easy. But generally, for his fixed blades, most of them will have tapered tangs.
The "Lum Style". This is harder to explain, much less describe. Bob had a very specific eye for lines. A trait that came from his skills as a photographer. With the grind lines, there is always a sense of flow with his blades. All the lines should just seems to tie in together.
Finally, the balance. Unfortunately, this can only be ascertained with the knife in hand. Bob had a way of making his knives balanced regardless of size.
So, based on the above, the following is what stops me from asserting that the knife is a Lum.
- No signature
- The grind lacks that crispness
- The blade's flat treatment is atypical of Bob's style
- The tip is atypical of Bob's tanto grinds
I'm hoping that we won't have too many examples of fake Lums on the market, as his work is not easy to duplicate. But with the prices of his knives at their current status, it's something we need to be wary of.