The 1909 S VDB cent is probably one of the most sought after American coins in the numismatics. Think about it; nearly all collectors either got started collecting lincolns or collect lincolns seriously sometime whilst engaged in the hobby.
image courtesy coinfacts.com
The problem with this is that there were only 484,000 of these coins minted. Considering the low mintage, the survival rate of this coin was probably fairly high due to the fact it was a first year issue and lincoln was quite a popular figure.
When you combine a great ever growing demand with a finite possibly shrinking supply you can deduce two things. There will be a lot of collectors that never get their 1909 S VDB and there will be a lot of collectors that purchase a fake 1909 S VDB. In a market where the price for even a specimen in good is constantly rising and quickly approaching $1000, it would be far better to be in the first column than in the second. But if you find yourself in the position to purchase a 1909 S VDB, I 1) recommend buying a slabbed coin from a reputable TPG, and 2) know what you are looking for diagnostically on the coin and how to tell the real thing from a fake because even the "experts" get it wrong from time to time.
The first thing to note is to get your self a loupe that is 20X or better just for looking at the mint mark. Because this is the first thing to focus on. The same S mint mark was used on the lincoln cent from 1909 to 1917.
image courtesy oicoins
Note the arrow pointing to the die chip inside the upper loop of the S. ALL genuine s vdbs will have this. There is a notch in the upper serif and the mint mark is of an even width at top and bottom, with perfectly parallel serifs. The upper serif is even with the lower curve of the S and the lower serif is even with the upper curve of the S.
There were four obverse dies used to create the 1909 S VDBs.
Looking at the 4 images above you can see the placement of all the mint marks in relation to the date. They all have the same stylized S mint mark as described above. I will not go into great detail about the mint mark locations as the pictures describe it better than I could. This is the best representation I have ever found with regard to the mint mark placements.
Just a note: Die 4 is by and far the most common of all the dies and is the easiest to attribute.
The last diagnostic to key in on is on the reverse of the coin, the V.D.B. It is not recommended to use the periods on the reverse to determine authenticity, because of the fact that some were struck weakly and the periods can not be made out. Rather, the shape of the V, the bottom half of the D and the last two bars in the B should be considered. The V is quite distinct the left side should be uniformly thick from the top to the angle while the right side should narrow noticably starting thick at the top and thinning as it reaches the angle. The angle itself should be sharp. The lower curve of the D should angle slightly upwards on a real specimen as the middle bar of the B should rise from left to right on a slant and the bottom curve of the B should angle slightly upwards.
I hope that this information serves you well when it comes time and you find yourself in the enviable position of purchasing the ever coveted 1909