how fast of FPGA that will think 100G optical port is bottleneck?

I understand each word of your question, but not the question itself.

actually i do not know which indicator define that it is the bottleneck when using this ethernet port when sending data from FPGA

I'm still not sure I understand your question, but the bottleneck really depends on what you put in the FPGA to control your Ethernet interface. If you are using a softcore solution, it will generally be the TCP/IP stack running on the CPU.