You need to use the debugger in FCEUX to find that out. Generally you follow a trail. For example, you see the pattern appear in PPU RAM, so you make a breakpoint for that to see where it's getting it from. Often it comes from somewhere in the CPU RAM, so you breakpoint THAT to see where THAT came from. Eventually you'll find something where the Accumulator is loaded from somewhere $8000 or more - that is, the ROM. FCEUX let's you go to that point in ROM and Bob's your uncle.
This is how I find everything in NES games, FCEUX makes things so much easier. The more you understand how the assembly works, the easier it becomes.