There is a difference between counterfeits, forgeries and fakes. A forgery is built from the ground up with the expectation of fooling the viewer into believing it's the genuine item. Surprisingly however, some of these very famous forgeries are of high value. A Spirati forgery of the Scott #2 is , for example is on eBay
now with a selling price of near $700). Scott Specialized has a section for Counterfeits, and the main differentiation here is counterfeits are usually a copy of an original in some way, and then mass produced (where forgeries generally have a small number). In my collection (of fakes, forgeries and counterfeits) I have a C15(CF1). Interestingly my counterfeit is of greater value than a MNH C15! Scott even lists a Plate Proof of the issue (in orange).
Fakes on the other hand are altered materials that attempt to LOOK like another stamp. While these are an abomination, we see them in almost every collection we acquire. Most will be aware, coils and "pressed out grills" are among the most common attempts at fakes. However, in many areas there are significant alterations that are made to "enhance" a stamps appearance. Some are obvious to detect with just a little knowledge (like the common attempt to pass off a 304 as a 315 (clip the perfs off). But we also see alternations to #7's and #9's in an attempt to make them appear as more appealing #5, #5A, #6, #8, #8A by painting in details, scraping away lines to mimic the desired characteristic. VERY careful examination under magnification (as well as plating) can identify such fakes. Another common fake method is to use proofs and essays as a means to create the fake. I have an example of a 149P3 that has added perfs to it to make it appear as a no-gum 149. (Realize the value of a 149P3 stock is $15, a 149 NG is a CV of $320). This one is VERY easy to detect, simply placed in watermark fluid, the design will shine through as the paper turns translucent in fluid. Once you get experienced, you can detect this by touch, as the India paper is thin, and has a slick, powdery feeling to the touch.
We take all of these out of circulation, but we have been amassing them as guidance in our book on fakes and how they are made. The more we "know" about what the stamp mechanics are doing, the better equipped we are to detect their attempts to defraud us.
Other alterations may include the addition of margins, or perfs. Some of these are incredibly skilled (I have no idea how they actually accomplish this), but with very careful examination in fluid and under magnification, they can be detected.
The rules of thumb are:
Always be suspicious of any piece that looks spectacular. Margins, vibrant color, high value... always question it.
And of course gum alterations are common as well, but you are probably all aware of those tricks.