Many people ignore the warning signs when it comes to their partner because they think they can do no wrong. Some can be ignored simply for the fact they aren't a big deal, but some can't since they are more serious.
Having friends of the opposite sex isn't a bad thing, but if you notice they are calling or seeing them late at night that can be a red flag that something else is going on. If your partner talks to people through social media networks, text messaging, phone calls, etc. and tells you all about them and that there is nothing to worry about and has nothing to hide, than you really have nothing to worry about, but if they start hiding everything from you or if you decide to snoop and see constant messages from the same person than that certainly is a red flag too that you need to worry about. If your partner starts going out without you when before you always used to go out together, than that is another red flag you need to keep an eye on.
These signs don't necessarily mean your partner is cheating (it certainly can be though), it can just mean your partner is living a double life. Which means they can either be a drug dealer, a drug addict, be part of an escort service, etc. If you are concerned about anything in your relationship confront them, but if you feel they are still lying to you after the confrontation, than you can either ignore it and stay with them or you can leave. Either way you decide to go, take care of yourself first, no matter what because your well being is more important than anything else.