How do we know that the God of the Sinai Revelation is the single God of All other God's? Is it not possible that whatever we encountered was simply a messenger of a greater force? I understand that there was a single infinite force from which the universe came forth, but how do we know that it is that force we entered into a covenant with?
This question is handled by the Rambam in the first few lines of the Yad.
As I understand it, basically we do believe there's a hierarchy - which is described as layers of angels - with the higher ones having more powers than the lower ones.
The very Highest Power - who is actually the source of everything - is the Being we treat as The God.
The first 2 commandments are:
[You must believe that] I am the One Who made everything and I am the Highest Power (lose translation of אנכי ה' א-לקיך)
You may not question this fact (based on the Rambam ibid 1:4)
וכל המעלה על דעתו שיש שם אלוה אחר, חוץ מזה--עובר בלא תעשה, שנאמר "לא יהיה לך אלוהים אחרים, על פניי"
A similar concept is reworded by the Mishna in Chagiga 2:1
וכל המסתכל בארבעה דברים, רתוי לו כאילו לא בא לעולם--מה למעלן, מה למטן, מה לפנים, מה לאחור
Essentially saying that questioning what is beyond the end of the universe - is forbidden. The mere discussion of a Greater Force than God is already forbidden.
(This is obviously a summary - the Rambam is more eloquent and more thorough.)
Rabbi Yisroel Chait tackles this question and others quite effectively in his article on Torah From Sinai. Article can be found here.
Generally speaking, we believe in things that we have positive evidence for, and we do not accept skeptical questions which do not have any positive basis to them. For example, we spend our lives growing up with our parents, being cared for by them, being told that we look like them etc. etc. This all amounts to positive evidence that our parents are indeed our parents. If a skeptic were to come and ask us for ABSOLUTE PROOF that our parents are really our parents, that we weren't say adopted or switched with a similar looking baby at the hospital, we would disregard his challenge as not having any basis. We have all of our experience to rely on in not accepting his position as true, and he has no positive evidence to back himself up with. However, if he would present us with adoption papers, we might start reconsidering. If he brought pictures of our parents at the adoption agency, we may start having serious doubts, but again he would need to bring very, very strong evidence to convince us to go against our intuitive experience.
The same is true for the Sinai experience. The challenge you present is essentially a "Bring me absolute proof" challenge, which is almost impossible to be met. But that does not give us a positive REASON to accept the challenge in light of our collective intuitive experience, which said that it was indeed God who appeared at Sinai. Additionally, it would be significantly difficult to explain the motivation of aliens to create as elaborate a system of laws as the Torah, as discussed here.
(as doubleaa pointed out in a comment) He said He is the only God. It does not make sense that a higher force would send a liar... and that liar would command us to be truthful in His torah etc. naaahhh