Here are some tips and tricks I've picked up from Jesse Schell's The Art of Game Design, and Tracy Fullerton's Game Design Workshop. If you feel the urge to delve deeper into this rabbit hole the information is on pages 256-265 in Fullerton and 390-401 in Schell.
WHEN IN DOUBT WRITE IT DOWN
The singular most important part of the act of playtesting, no matter what method is used, is note taking. We'll get into specifics in a minute, but if something occurred during playtesting that seemed important write down what happened and when it occurred during gameplay
DO NOT LEAD THE PLAY-TESTER
If the play-tester at any time becomes stuck or too confused to move on during game play two things should happen:
1) It should be written down. This tells you something about this part of the game needs to be fixed.
2) Don't tell them what to do. If this occurs ask them what they think they should do and then have them try their solution. Also remind them to think aloud.
Note: This second one is extremely difficult and failure to comply with it may at some point be understandably unavoidable. Just do your best to lead the tester as little as possible.
Now, as to not just rewrite all of the subject matter Schell and Fullerton already wrote, I'm just going to give some examples of the types of questions that should be kept in mind while play testing
- Are players:
Playing the way that was intended
- Is the game too
- Do players understand how to play?
- Is level 3 too short?
- Do players have enough to do?
- Do players have too much to do?
- Are players over whelmed?
The list can go on and on, but the most important thing to remember remains:
WHEN IN DOUBT WRITE IT DOWN.