How will I know if someone is monitoring my phone?

Could you please explain how I might detect if someone is spying on my phone? I'm quite concerned about my privacy and want to ensure my device is secure. Are there any tell-tale signs or specific checks I can perform to verify the integrity of my phone? Would changes in battery usage, unexpected data consumption, or odd behavior of apps be indicators? I'm also interested in knowing if there are any reliable security apps that can assist me in this matter. Thank you for your assistance in helping me safeguard my personal information.