Hacked sites will frequently experience a dramatic spike in traffic because your site becomes the "infection vector" for visitors which are sent specifically to your site to get malware installed on their machines. BYour might also discover strange links on your site, content which you have not written, or get messages from your WordPress hosting site and possibly even the Google Search console. If you start seeing strange things on your site, or significant performance degradation, or other issues that you can't put your finger on, it's a good idea to speak to a security expert.