Do you mean, Juliet, then if you wrote down their speech without tags, it would be difficult to tell who was talking?
I had this problem and to remedy it thought really hard about the vocab i was using.
I noticed, eg that my 40 year old female lead and 30ish male lead both said similar things - eg
"I don't mind"
I thought about it and the male would probably really say
"yeh, whatever" or "cool"
Think about every single word/each phrase each character says
would they say 'bye' or 'goodbye', 'see ya' or 'ciao'
would they say 'how are you?' or 'you okay?' or 'What's up?'
you get the picture.
I went through my book about 5 times, solely reading the dialogue of a particular character each time, making sure that each line of theirs was in keeping with their voice.
Good luck