You can

• Watch network access - What is communicating to the outside? Why is it? What is it communicating? I like sniffnet
• Watch CPU and GPU usage - What is using resources? Why is it? I like Mission Center and ps auxf in the terminal
• What is using the disk? I like Filelight and Disk Usage Analyzer
• Run AV software. I don’t have any recommendations.
• And I guess sometimes you can also find it when benchmarking software because you want the absolute best performance.

You can also assume you are compromised and use a solution like a Faraday cage. If you’re trying to detect advanced spyware, it might be better to check network activity from outside the device like what network activity is the router managing for the computer.