Here are the tips (not that I went crazy and did all this!).
If the ipad is linked to his iphone, go into the settings and there is an option for accounts and passwords. If he has set passwords to be remembered on his iphone it will show you each account and the password for them, so you can just log in to his account (watch out as he may get a 'your account has been accessed' notification if for his email/facebook account).
Check the pictures and drive on the ipad. Again they will be linked to his iphone, so if he has photos on his iphone they will also show on the ipad. Particularly watch out for any messenger, whatsapp and recently deleted photos shown.
Does he have a google email address? If so on google maps there is a timeline option. If it is set (if it is not you could log in to his email address and activate it in settings - you do not need his phone to do that - but he may get an alert) then you can log in to google maps with his username and password (it may show in the ipad settings) and then click on timeline and it will record where he has been, giving you a map and everything! It is quite creepy, but it can help if he suddenly starts appearing at someone else's house for lengths of time.
Whatsapp is the usual source for cheaters, but it is hard to get access to unless you can get onto his phone. You can link his phone to a web browser to keep an eye on conversations, but that is a step too far.
I have been where you are and you will feel like a complete stalker, but it is apple's fault for making it easy to get settings from the ipad and only the reassurance either way depending on what you find will ease the feelings.
Alternatively, you can talk to him, but if he is playing away then he may cover his tracks and you may find it hard to get to the truth.