If your lover is telling you one too many lies about serious things of life then chances are high that relationship will bring a lot of pain and suffering in your life, even if you two end up marrying!
Honesty is an essential part of lasting relationships, and often a small skeleton from the cupboard destroy a relationship after years, so if you believe that your lover is lying and you are allergic to too much lying just tell that you believe honesty is must.
Now, there are harmless lies like s/he forgot the anniversary and then told you s/he was sooooooooooo sick that s/he forgot his or her own name for the day(s).
Then there are harmful lies like having another lover, being jobless, being a criminal, a murderer or a fugitive… or worse, being married!
If you ask me I will say that you clearly say your lover, with whom you are planning to spend life to be absolutely truthful about serious things, you won’t suffer treachery, dishonesty.