How can I find out if my spouse is cheating on me?

How can I find out if my spouse is cheating on me?

Hi friend, my spouse never lets me access any of his emails, messages on mobiles and whatsapp. Somehow I peeked into his mobile phone and found messages from his ex-girlfriend. Is there any app available I can use to spy my husband's phone? I want to monitor his messages, whatsapp chats, call logs, etc. Help me please.