Someone else is giving testimony about me, and I have seen that the evidence that he reports about me is exposed.
Jhn 5:32 There is another that beareth witness of me; and I know that the witness which he witnesseth of me is true.
Interesting and Hidden Aspects:
Christ is explaining something rather complicated (see wordplay section below) here that is easy to miss unless we look at the Greek content of this verse.
In several previous verses, most specifically Jhn 5:27, Christ has been claiming that he has been given authority and power, specifically the power to judge. In the previous verse, he said that he could not make these claims for himself.
He begins this verse as something of a mystery. Someone else, not yet mentioned, is giving evidence of these claims. He says that he personally makes these claims only because he has seen the evidence provided by another.
The word "see" here is very important. Though translated as "I know," the Greek word Christ uses means "to see." It can mean "I know" in the same way we might answers someone "I see" to explain our understanding. However, here, Christ is talking more literally about seeing. We know this because of the context.
What is he seeing?
The answers is the word translated as "true." Again, the Greek word is different, meaning literally "unconcealed." We might say "exposed" or, in a more Biblical tone, "revealed." Christ is seeing the truth that has been exposed. You see what is exposed, you don't know it.
What has Christ seen exposed? The evidence that is given by the mysterious source mentioned in the first part of this verse. In later verses, Christ not only explains the source of this evidence, but why others cannot see it.
Christ plays on the idea of witnessing ("seeing") the exposed witnessing of a hidden witness.
ἐστὶν (3rd sg pres ind act) "There is" is from eimi (eimi), which means "to be", "to exist", "to be the case," and "is possible." (The future form is esomai. The 3rd person present indicative is "esti.")
ὁ μαρτυρῶν (part sg pres act masc nom) "Beareth witness" is from martyreo, which means "to bear witness", "to give evidence", "give a good report", "testify to," and "acknowledge the value of." It is the basis for our word "martyr."
καὶ "And" is from kai, which is the conjunction joining phrases and clauses, "and," or "but." After words implying sameness, "as" (the same opinion as you). Used in series, joins positive with negative "Not only...but also." Also used to give emphasis, "even", "also," and "just."
ἀληθής "True" is from alethes, which means "unconcealed", "so true", "not forgetting", "careful," [of persons] "truthful" "honest," [of oracles] "true" "unerring," and [as adverb] "actually" "in reality."
ἣν "Which" is from hos (hos), which is the demonstrative pronoun in its various forms (hê, ho, gen. hou, hês, hou, etc. ; dat. pl. hois, hais, hois, etc. gen. hoou). It means "this", "that", "he", "she", "which", "what", "who", "whosoever", "where", "for which reason," and many similar meanings.
μαρτυρεῖ (3rd sg pres ind act) "He witnesses" is from martyreo, which means "to bear witness", "to give evidence", "give a good report", "testify to," and "acknowledge the value of." It is the basis for our word "martyr."