An ADHD person?
Because I had a partner who was a sociopath, wow, you have never seen anything like it. Absolutely no social conscience at all.
The way to tell is if they lie knowing they’ll be found out. Look up sociopath and you’ll see common traits.
If they match, then end the relationship! I mean it!
If it’s adhd lies then just have a chat and ask not to be lied to, think before they speak or even walk away as the lie starts. Give them a way out.
They should be mortified you’ve raised it and want to make things right. ADHD peeps hate lying usually. Just read the comments here!