I don't work in a big game studio so I cannot say this for sure, but I don't think there is a certain written-in-stone system on how to playtest
games if that is what you are talking about.
In my opinion though, even when playtesting games (that is, just playing them and taking notes) does require some eye for certain things, so having even some experience of developing games by yourself would be a big plus.
I think the most important things when testing a game is:
- Try to go through all of the features of the game and do things in an unexpected way, if the game tells you to do something, try to do the opposite. It might just be that there is a bug lurking out there somewhere.
- Pay attention to the controls and the difficulty curve. When certain people work long and repeatedly on a game, certain things start to feel the easy and natural way to them. Someone who hasn't touched the game before can see things in a different way and give constructive feedback to those who are already "blind" to the flaws in the difficulty curve, control scheme or things like that.
- Often the developers of a game are concentrated on the technical side of things. However the purpose of games is to work well, but also entertain. List out things that you think could make the game more fun. Programmers are usually concerned about the technical limitations and such, you as a play tester shouldn't be though. It's possible that you can come up with new and innovative ideas to improve the gameplay that aren't that hard to implement after all.
I am afraid that I cannot do much to help you in your original problem. That is, helping you to be able to say you have tested (a professional) game. However, if you want to help me out, you can try a game of mine that is under development. You can check it out in this topic