perf12, I actually don't see a lot of fakes that have the design modified/ drawn on. I'm sure it can be found on almost any old issue. Obviously higher priced items are more likely to be faked, but not always. One should be somewhat cautious about stamps with recut line (for example #23, #16, #34). I'm sure there's copies out there with a drawn on line, attempting to pass as recut line.
Another thing one needs to be careful about is drawn on guidelines/joint lines; found on coil issues. I've come across a few that were drawn on. One must look very closely at the color & ink type. I've even seen them drawn on coils that weren't very expensive/rare.
Typically you won't have to worry about stamps being drawn on. Fakers do anything to try & make something believable. You're more likely to come across trimmed & fake perforation stamps; trying to resemble a rarer stamp.
Here's 2 examples of perforated Scott #35 Type V, currently listed on
ebay. The perfs have been trimmed away, in an attempt to resemble the more expensive Scott #31 Type I. One can tell they're both trimmed by how incomplete/short side ornaments are & there's no pearls visible on the sides, heavy bottom line, etc. This is quite common.


Here's what they originally looked like:
