Highlight all the dialogue by your lead character. Use a different color to highlight all the dialogue by the second most important character, another color for the third most important, and so on.
Note the proportion of highlighted material to unhighlighted material. Remember that readers love dialogue. Some readers even skip narration to get to it.
Also, the proportion of dialogue for each character should be in keeping with that character’s importance, unless you have a character who is taciturn or loquacious.
Now read only the lead character’s dialogue. Make sure the voice is consistent and distinct. Do the same for each of the other major players. As a further test, eliminate all the dialogue attribution (he said, she said). You should be able to tell whose dialogue it is by the speaker’s diction and syntax.