It would be possible to fake bike components, but probably not cost-effective. You shouldn't have to worry about fake 105s. If I were going to fake a component group, it woud be Dura-Ace or Super Record. Then I'd just take a lower end component group from the same manufacturer and change the graphics and maybe do a little paint of polish job to make it look more like the high-end stuff. I would never do this of course; just postulating.

I bigger concern when buying used is components that have been ridden thousands of miles and/or not maintained properly. To learn the difference in looks, sounds, and feel between worn out components and stuff that still has a lot of life left, you have to check out lots of used bikes. Going to swap meets is a lot of help in that regard. It would be a good idea to have somebody who rides a lot and works on his/her own bikes go with you to check out a used bike if you are going to make a serious investment. Kind-of like having a mechanic check over a used car before you buy it. What you consider a real investment is up to you. For some folks, $3000 for a bike is no big deal; for me $300 is a lot.