Some methods the scientists could use would be, monitoring the population of prey which the lionfish is known to consume, and also monitor the size/weight of the lionfish in order to understand how and why they are able to flourish.
Cell - Different cells make up organs
Cell Membrane - Protect the cell
Cytoplasm - Contains all organelles and cell parts
Nucleus - Contains the cell's genetic material
Nuclear Membrane - Allows small molecules, ions, and/or proteins to move in and out of the necleus
Ribosomes - Where RNA is translated to protein
Endoplasmic Reticulum - A manufacturing or packing system for the cell
Golgi Bodies - Gathers simple molecules and combines them to be more complex
Mitochondria - Energy factories of the cell (Side note: Always remember "Mitochondria is the powerhouse of the cell")
Lysosomes - Digest excess or worn organelles, food particles, and viruses or bacterias
I believe vestigial structures are evidence of evolution because organisms inherit anatomy from their ancestors.
Hope that helps.