You can find this by using console commands.
All of the below are helpful at isolating causes or at least getting you close to a solid theory.
Stat raw
stat initviews
Profilegpu
Isolate your bottleneck and optimize or make changes to code in those areas. Closing other programs,apps and unneeded windows in editor help close blueprints and disable any popup overlays in any programs or background tasks,anything that draws attention from the rendered window can have horrible effects.
Play test iteration and framerate profiling come near the end of the development cycle, not during it. You need to lookout for things like transparency overdraw, over complex materials, exaggerated particle emitters,nonexistent LOD meshes and poor static mesh culling. Having a good idea of the tradeoffs during content creation pipeline will allow you to avoid most problems. Too many concurrent tick events throughout the game can bog you down too.
Post some photos of your project trouble areas they sometimes speak louder than words.