While the ostensible purpose of a debate is to further educate the American public on the stances of the candidates, most viewers - both among the general public and the media - watch to determine one thing.
The answer will be subjective, and - for many of the die hard supporters of either candidate - essentially moot. Mr Trump's base will claim he won, no matter his performance, and Mr Biden's base is likely to do so as well.
But winning a debate isn't necessarily just about swaying the small number of undecided individuals who might be watching. A debate win in the eyes of the public can embolden a candidate's base, drive voter turnout and - perhaps most important for a campaign - entice donors to pump more money into a candidate.
Though the popular answer to "who won" will largely be decided based on the response from the voters and the subsequent media coverage, there are some tell-tale signs to watch for during the debate to shed some light on who is winning, and who is reeling.
Dr. Tammy Vigil, an associate professor of communication at Boston University and an author of several books about presidential debates, campaigns, and rhetoric, said that watching candidates' nonverbal cues can provide a glimpse into how they perceive their performance on stage.
According to Dr Vigil, a candidate's posture will often telegraph how they're feeling during a debate.
"Watch for nonverbal communication. A candidate that's standing straighter with their head up is likely feeling confident," she said. "You can tell by the way a person is carrying themselves if they're feeling confident."
Posture isn't the only tell; a candidate's eyes can also provide insight into how they're feeling on the debate stage.
"Looking down, looking around - these can betray a lack of confidence in a candidate during a debate," she said. "It makes people look uncertain."
The opposite - maintaining focus and eye contact - produces the opposite effect. Dr Vigil said that candidates who are well trained on the debate stage will actually keep eye contact with the cameras, as it can produce feelings of connection between the viewers and the candidate.
Because everyone is different, verbal tells can be a bit trickier to analyse. However, there are a few broad insights viewers can make based on the way a candidate speaks.
Dr Vigil believes verbal fillers - utterances like "uhm" or "uh" - that break the momentum of a candidate's point can suggest a lack of confidence in a candidate. But she warns that those fillers aren't always indicative of a reeling candidate, and can just be an effect of the way a person naturally speaks.
Clear, concise answers are another verbal cue that can indicate a candidate feels like they've got control of the stage.
"Verbally, when someone can provide real information in a concise and clear manner, it suggests that they've prepared," Dr Vigil said. "That usually means they feel that they're doing well."
Candidates scrambling for answers or pausing mid-sentence can telegraph to the audience that they don't know what they're talking about, or suggest they hadn't prepared for the question they're answering.
Dr Vigil pointed out specific ticks each candidate has that can provide a window into their mentality during the debate.
She said Mr Trump tends to lash out when he feels cornered or challenged. While Mr Trump is always aggressive and bullying, she said that his tendency to jump from topic to topic and to attack his opponents becomes more frenzied and less coherent when he's on the back foot.
"Some people have compared it to the way a small child lashes out when they're being challenged. He'll change the subject, resort to things like name calling or making things up," she said. "You can tell because his demeanor shifts; he'll look down and begin scowling."
If Mr Trump is feeling confident however, he's still likely to be aggressive, use insults to bolster his message, and to use choppy, disconnected sentences, but he'll also repeat himself and generally tend to stay on topic.
When Mr Biden feels backed into a corner, Dr Vigil says he'll tend to use more vocal fillers, and break eye contact with his opponents and the audience. He'll laugh while his opponent is speaking and sometimes become angry.
If the former vice president is feeling confident, he speaks with more of a flow, and uses the catchphrases that have become a staple of his political personality.
"He uses more catchy phrasing when he's on a roll. When he's confident, he's got a better flow to him. He's used odd turns of phrase like 'malarkey' that may sound hokey and out of date, but it's a sign that he's feeling comfortable."
The first presidential debate will air on 29 September and will take place at Case Western Reserve University in Cleveland, Ohio.