How can we know that their teachings came from God?

Well. It's just easy to know it. We already know that the words of God are written on the Holy Bible, and therefore those teachings that go against the Holy Bible are not coming from God, and those teachings that go in parallel with the truth written on the Bible are the teachings of God.

Now, let us ask the Holy Bible: Is it true that the Bible can't be the sole guide to Salvation? Apostle Paul answered:

15 and that from childhood you have known the Holy Scriptures, which are able to make you wise for salvation through faith which is in Christ Jesus. (II Timothy 3:15, NKJV)

According to the Apostle Paul, the Holy Scriptures or we call it the Bible, can make us wise for salvation. In other words, the Bible can lead us to salvation. But how could the Bible lead us to salvation? From that verse, Apostle Paul said that it is through our faith to Christ.

But they said that the Holy Bible does not contain the complete truth about Christ. So, how can we believe Christ if we do not know who He is and what He is?

It is true that Christ did many things and many of those things were not written down, because according to Apostle John, "if every one of them were written down, ...even the whole world would not have room for the books that would be written" (John 21:25, ibid). But, are the things recorded on the Holy Bible enough for us to believe Christ and gain salvation? Apostle John said:

30Jesus did many other miraculous signs in the presence of his disciples, which are not recorded in this book. 31But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name. (John 20:30, ibid)