Playtesting advice

Hiya. So I'm fairly new to the world of homebrewing -- I've always done a bit for my own campaigns and helped my players tweak things here and there but fully designing new classes etc. is unfamiliar territory. As I've been doing more and finessing my ideas it's got me wondering how do most people manage playtesting? Obviously you can just build a character and run it with a friend but since most TTRPGs are so in-depth and time consuming it doesn't feel like the most efficient approach, especially for something larger like a class or domain. Does anyone have any techniques for playtesting more efficiently? Or ways to do it solo?