The fact that your husband may have a friendship with a female coworker or family friend does not mean there could automatically be an affair between them. Some men are capable of keeping things at the friend level and never going beyond it.
The next level could be an emotional affair with a woman where there is no physical contact but your husband could feel closer with this person and be able to tell all to them. They may feel more comfortable talking with this woman about personal problems than with you.
What signs could you see if there is some kind of affair taking place with your husband. If your husband used to talk about a certain woman friend but suddenly she never comes up in a conversation be aware, especially if you know she is still around. The relationship between these two could have taken a more serious turn. It is not unusual for a couple having an emotional affair to take it to the next step above that.
Another sign you should watch for is how your husband behaves to you. Is he becoming more withdrawn from you. This could because his feelings for the other woman make him feel guilty when he is around you. He could also find it difficult to be in love with two woman at the same time and does not know how to handle it.
If you find there is in fact an emotional affair taking place take immediate action on it. The chances of surviving an emotional affair are great. There may be damage to repair but ones marriage can be repaired.