Kiss their neck
Both sexes can be tickly on the neck and when you give them a kiss they might jump. This can actually be sexy and you have nothing to worry about. When they are playing on their phone and they jump before you even reach their neck it’s then you should start worrying. Nobody with something to hide is going to keep a straight face when they are texting another person, especially when you’re breathing down their neck.
Watch their habits
People have strange habits and when they suddenly start deviating from them it can be quite strange. If you were never paying attention to your other-half you might not have any idea that something was up, but when you’re already suspicious and they suddenly start coming home from work an hour late there must be a reason for it. Hopefully it will be a good reason, but not always and you’ll be able to probe more to find out. All you need to start doing is paying more attention to simple movements.