I was wondering if we can actually "know" the true pronunciation of any Hebrew name. For instance, is it like today someone names their child "lemonjello" pronounced "lay mun cha loo" not like "lemon jello."

I was wondering since I understand there were no vowels like in modern English. Is this correct? If so, are the Hebrew pronunciations just our best guesses?

You are correct that original biblical Hebrew was written without vowels. People learned how to pronounce words verbally from generation to generation. Centuries after Jerusalem was destroyed in 70 AD and the Jews were scattered among the nations, scribes realized that the pronunciation of Hebrew was being lost because Hebrew was no longer a Jewish children grew up speaking. They came up with a system of vowel pointers that allowed them to record the vowel sounds without having to alter the actual Hebrew text (the marks are place above and below the letters in the text). Hence the sounds have be preserved for us since about 900 AD.

However, we have a secondary source for pronunciation. The Hebrew text has been translated into other languages. Names especially are moved over in a transliterated form (the word sound remains the same, but it uses the target language's spell to approximate the sound). Because of this we can see how names were pronounced further back in time. For example, the Septuagint version contains Hebrew names transliterated into Greek. There are also old Arabic translations available.

Sometimes we also find old commentaries on the Hebrews Scriptures written by Jewish rabbis, but written in the language of their region. Again, the transliteration helps scholars learn how the names were pronounced at that time.

Is it 100% accurate? No, because there are always regional dialects. Just has American English sounds different between New England, the Old South, and the Midwest, there were regional dialects of Hebrew. An example of this is found in Judges 12:5-6, "The Gileadites seized the fords of the Jordan before the Ephraimites arrived. And when any Ephraimite who escaped said, "Let me cross over," the men of Gilead would say to him, "Are you an Ephraimite?" If he said, "No," then they would say to him, "Then say, 'Shibboleth'!" And he would say, "Sibboleth," for he could not pronounce it right. Then they would take him and kill him at the fords of the Jordan. There fell at that time forty-two thousand Ephraimites."

Suggested Resource:

Pronouncing Bible Names by W. Murray Severance