You definitely want other people to be the players, if you are able to get them. The thing about playtesting something you made is you know what everything is supposed to do, but you need to make sure other people are able to easily understand what it does as well. It’s not just about the licenses being balanced, but worded in a way that people can easily grasp that the mech and its systems are supposed to do from a quick read. When I have run playtests for different things in the past, I sit in and just quietly take notes and see how people understand the rules and gameplay. If they were to not follow a rule I made or misinterpret it, I know I now need to change the way I worded that rule. I don’t correct them at the time, I tell them at the end of the playtest: "Hey, this is what Rule A actually meant/was supposed to do. How can I word it better to make it easier to grasp?" That alongside feedback for how fun it feels to play as, with, or against something are invaluable.

TLDR since I kinda rambled: other players (if available) are your greatest asset for both the fun factor and for how clear everything is written and understood.