1. Guarding The Cellphone - It's not uncommon for most people to set a password or some form of lock on their phone. When you start to see your spouse guard their usage say, frequently locking themselves in the bathroom for long periods of time or only using their phone when your across the room, is a good indicator there is some untrustworthy activity going on. You can always test this by asking a direct question, if they don't answer yes or no immediately, it is a good indicator there is some deception going on.
2. Begins getting distant or making hurtful remarks - This is done in hopes they can create distance from you. Picking fights like this is an excuse to say you are always fighting as well, or unnecessarily playing the victim.
3. Doesn't want sex - Do not kid yourself if you do not have a sex life with your partner. If they are not getting it from you, then it is either only a matter of time or it's already happening that they are getting it from somewhere else.
4. Taking out credit cards in their name - If its unnecessary, then it may be an attempt to cover up expenses on a potential partner. Look out for other financial avenues that would allow for a deceptive lifestyle.
5. Gut instinct - When you find your spouse no longer comes to you for advice or to just talk about life. Maybe you get a hunch that your mutual friends act different because they might know something is going on. You see a difference in a pattern of habits such as they suddenly act different, dress different, and become a different person almost.
Not in all cases what I defined above means someone is cheating, but in my experience, they MAY be indicators that perhaps there is some deception. There are many ways to check for deception through a series of tactical questioning. The biggest advice I have for anyone is that no matter if you are being cheated on or not. It is always best that you protect yourself as much as possible: financially, emotionally, etc. Ask any divorcee and they will agree that they never planned on getting divorced. Always seek a strong bond with your partner, but always seek to protect yourself if you find your with someone who doesn't have your best interests.