I've been searching around Google Scholar (e.g., https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2869.2009.00767.x, and https://psycnet.apa.org/doiLanding?doi=10.1037%2F0894-4126.96.36.1997), and studies don't seem to mention what methods are used to keep participants awake. Are there any sources where I could find this information, or does anyone know? It seems particularily important to control for possible episodes of microsleep.
This is necessary information to include in the methods section of a paper. You should assume they've done what they say they've done and nothing else.
For the first study, it doesn't seem like they did anything except tell the participants to stay awake. They refer to use of an "actiwatch" to make sure no one napped between their sleep and arriving at the laboratory, and then they watched them:
For the TSD condition, they were kept under constant supervision for a further 25 h to ensure that they remained awake
This is a fairly mild sleep deprivation, so it doesn't seem like they needed to do anything else.