WhatsApp User & developer, engineer of IT systems, many like WhatsApp.
The decent and LEGAL way:
Telling her directly that you would love to read her private conversations and thus asking her for her phone (device) and asking her for her password to access the phone and possibly the PiN to access the WhatsApp app in that phone.
She may or may not accede to your request.
If you have any doubts or questions about what she comunicates and what she does, ask her !
If you don't trust her, then thats a very sick relationship.
Of course, there's the indecent and ILLEGAL ways: syealing and cracking her phone, or secretly installing spyware or cracking her WhatsApp credentials and stealing the information… But all this requires sufficient tech skills, could also land you in jail as it's a Federal crime in many jurisdictions (wiretapping) and surely won't solve any issues you could have with her.
20 years in IT
Ask her. If you don't trust her you should not have married her. Either you are insecure or she has given you a reason to try to play amateur detective. In either case once you feel this way the relationship is on a downward spiral. If you found something, then what? Confront her? You won't ever be able to trust her after that. I think you have larger issues than WhatsApp of you feel the need to spy on your wife.
It Technician (2009-present)
Fuck decent and legal, lets look at ethical. How would you feel about your wife spying on you? How would you react to that blatant lack of trust? What if it wasn’t your wife, but your boss? How would you feel about me, a complete stranger, reading your WhatsApp messages?
A relationship is based on trust. Either trust your wife, or work on your relationship!
Msc Telecommunications Engineering, Polytechnic University of Tirana (2016)
You can't and you shouldn't. This is the simple answer.
There are some ways which give you illegal access to another phone (which require physical access first and superuser rights to work), but since you are asking this question you cannot do it and furthermore it's not something that all end users can do, so better not to try it and wreck and brick the phone.
If it were that simple I'd be reading your messages now.
If you doubt anything, better ask her than ask to hack her phone.
VoIP Solutions Architect at Wanet Telecoms Limited (2010-present)
Install Xymen for remote Whatsapp in your phone. Add client & open whatsapp on your wife's phone, tap Whatsapp Web and scan the barcode on your Xymen app. Check stay logged in on your Xymen app. At this point you should be able to to view her Whatsapp messages as long as you both have internet access.
Simple as it sounds. Just get her phone unlocked and see it.