28f think partner 32f is lying about texting their ex - how can I know for sure?

Pretty sure my partner of 18 months is lying about messaging their ex. They have been caught 3 times previously. I think they have the the ex’s number saved as another name. Have outright asked if it’s them and been told no, but tried fb ‘forgot password’ using the phone number to see linked email and first letter/end of email looks like could be the ex who I’m concerned about.

Any way to check who a number belongs to? UK based.