Since you presumably know your partner better than anyone else, any significant change in their behaviour should be cause for concern.
When lying, it's usual for people to alter their behaviour by making their phone more private, even by "hiding their phone from you.
When you begin to suspect that your partner isn't being completely honest with you about something, you're probably right.
Since it is said that the eyes are the window to the soul, if your partner is hiding something from you, they won't want to show you theirs and will begin to avoid making eye contact with you.
When your partner begins a sentence with the words "I'm not going to lie" or "to be honest," it may be a clear sign that they're actually acting in opposition to your wishes.