You should check this talk at BlackHat 2014 and this one from Bunnie & Xobs at 30CCC.
I could go about some details but it's so vast, it is meaningless to try to explain the details of reversing a chip here. Decapping can be helpful if you're willing to probe the IC using needles and an oscilloscope and reverse engineer a protocol or extract data; or use a microscope to take snapshots which you could compare to some already existing chips, ... Here's a very nice Hackaday talk with very interesting links and techniques.
I would recommend getting the data sheet of the chip first -if you can identify it- or check out the manufacturer on-line to see if there's any info about the end product in general that could lead to identifying the components or the manufacturer of the component; or if anybody tried to tinker with the same product or found that same chip in another product.
Use the Chinese search engine, most of these chips are made in China and, for example, Baidu can return some helpful links and even documentation and tools.
Bear in mind that sometimes unmarked chips are knock-offs of well known chips, you can check a known Flash manufacturer data sheets and compare the pin positions and even probe the pins using an oscilloscope to see if they match the data sheet description.
If you have any deeper technical questions about the output of a process/analysis I'll be able to help more. You need to workout a set of thorough steps before you dive into reversing a chip because it can get VERY confusing at times.
Helpful publication 1
Helpful publication 2
Helpful publication 3
Helpful publication 4
Check out these guys too : link, they have developed a whole hardware platform for hardware reversing. Its quite expensive (700€ ~ 680$) but very handy, especially the chip database.