My husband has an iphone with touch id. I was wondering if anyone had any suggestions as to how to subtlety scratch/ruin the button without it being obvious. Also I don't want to run the risk of otherwise harming the phone. I think I could get away with a small scratch but otherwise he would suspect something was up. Maybe there is something clear I could put on it to smudge it.
The idea is that if he can't use the touch id, he will be forced to enter the 4 digit lock code. As he does this many times a day, eventually I will be able to observe him do it when be doesn't know I'm looking.

Not an answer to your question, but other monitoring tools:

Is he using WiFi or cell phone data? In case of WiFi, you can monitor the internet use via the modem.

Can you slip a VAR in his car?

Is there a specific reason you want to snoop (has he changed or do you suspect something is off) and can we help you with that?

A four digit lock code is not that hard to crack. Take a look here https://www.popsci.com/technology/a...day-fastest-way-crack-4-digit-pin-number

But when I wanted to get the goods on my cheating now XH, I found a backup and used that.

In addition, as goody suggests, get a VAR and put it somewhere where he is likely to be making calls.

Turn the phone off. After power up, you have to enter the code because Touch ID won�t work.

I could turn off the phone every now and then but if I do it too much he would get suspicious. I can't try guessing more than one or two codes at a time because he has it set to delete the phone after too many failed attempts due to it having work data on it.
I'm not trying to see his calls. We have verizon and I am the account owner. I can already see his calls and sms texts. I mainly want to see his non sms (imessage) texts and emails. He doesn't use a home computer so that is not an option. He pretty much uses his phone for everything.
For the record, I do not think he is having a physical affair but I do think he was getting close to an emotional affair a while back. This was something I came across by accident. I do not think it even progressed to sexting or anything like that - just a "friendship" that was getting too close to the edge. He doesn't even know I know about it. I don't think he is even in contact with her anymore and from what I read they had never met but I want to keep an eye on things. Things are actually quite good at home now but unconditional trust is now a thing of the past. Trust but verify is my new motto.

Trust but verify is great, but the goal should really be to have a marriage that is affair proofed. Transparency is paramount in affair proofing, he should not have devices, social media, etc that you do not have access to.

I would advise you to continue snooping to rule out an affair. But if there is no affair (rule this out first), to discuss with him your need for transparency. There is no reason for secrecy unless he has something to hide afterall.

I would agree with unwritten.

I was in a similar place and did find there was an affair going on.
Like you I had suspected an emotional affair back in March and I asked him no to communicate with her anymore. He agreed and did not, however he felt I was controlling him and rebelled with another women. Having an emotional and part physical affair (fondeling) with another women. I found out by using 'Wondershare Dr.Fone' while he left his phone unattended. I used this because I has access to his phone but he was deleting them to not get caught. This program recovered the deleted texts.

Continue snooping to get the truth/facts if he is having an affair, but hiding and deleting messages from you is not any kind of relationship to have.

I feel for you, I have been in your shoes.

Hi mamabear, I hope you start up a thread on the Surviving an Affair forum so we can help you. It sounds like your husband is very manipulative to blame YOU for his affair. He is actually controlling YOU by inflicting his abusive behavior on you. You are not "controlling" him when you ask him to stop abusing you. His manipulative behavior is very alarming and reflects a wayward mind.