There is a whole set of surveys in "Not Just Friends" that allows you to evaluate marital, individual, and social vulnerability. Some of these can be controlled and changed, some cannot. Even with good conditions, you cannot affair proof your marriage/relationship.
You and your spouse should fill these out.
Marital
(1,2,3: Disagree, Agree Somewhat, Strongly Agree):
1. We had problems trusting each other before we got married.
2. Our Marriage revolves around our children. OR (for childless couples) We disagree on whether or not to have children.
3. My partner spends too much time away from home.
4. My partner rarely takes my side in anything.
5. We've grown apart.
6. I have felt alone and unsupported at times of loss or crises.
7. We don't have equal input for important decisions.
8. We argue about the frequency of sex.
9. Our interactions feel more like a parent-child relationship than one between equals.
10. We are uncomfortable about exposing our inner selves to each other.
11. We sweep things under the rug, so we hardly ever fight.
12. There's a disparity in how invested we are in the relationship.
13. I feel I can't influence my partner to do what I request.
14. I don't know if I really love my partner.
15. We don't know how to repair after a conflict.
16. We don't have much in common.
Score:
16-20 = Low
21-29 = Medium
30-39 = High
40-48 = Danger
Individual:
(1,0: Agree, Disagree except Part II where 0,1 : Agree, Disagree)
Part I. I would feel justified having an extramarital relationship for the following reasons:
1. If I fell in love with another person.
2. For sexual excitement or sexual variety.
3. To have someone understand my problems and feelings.
Part II. I would be inhibited from having an extramarital relationship for the following reasons:
4. I would feel too guilty.
5. It's against my moral, ethical, or religious principles.
6. I wouldn't break my wedding vows or my commitment to my partner.
7. I'm devoted to my partner.
Part III. I'd probably get involved with an attractive person who is interested in me because:
8. I work hard, so I deserve to have some fun.
9. I wouldn't be hurting anybody.
10. It's okay to have sex as long as you don't get too emotionally involved.
11. It's okay to fool around as long as you don't have intercourse.
12. It's okay to be emotionally involved as long as there's no physical intimacy.
Part IV. The following statements reflect my beliefs about life:
13. I thrive on risky, new adventures.
14. Rules are made to be broken.
15. Forbidden fruit is sweeter.
16. Beginning a project is always more interesting than finishing it.
Part V. The following statements reflect how i feel about my relationship:
17. I feel uptight if my partner tries to get too close.
18. I feel anxious and rejected if my partner ignores me.
19. I'm most comfortable being alone. I have little need to spend time with my partner.
Score:
0-4 = Low
5-9 = Medium
10-14 = High
15-19 = Danger
Social:
Quiz (1, 0: Agree, Disagree):
1. I have to travel a lot for my work.
2. My work offers me the opportunity to interact with attractive colleagues whom my significant other might perceive as a threat.
3. Nobody at my workplace would condemn me for an "office romance".
4. I enjoy going to Happy Hour with my friends or colleagues.
5. I have friends who confide in me that they are cheating on their partner.
6. I have lots of fun with my single friends because they like to party.
7. Most of my friends would be understanding if I cheated on my spouse.
8. Hardly anybody I know is strictly monogamous.
9. There is a history of infidelity in my childhood family or extended family.
10. My father would support me and not condemn me if I cheated on my partner.
11. My mother would support me and not condemn me if I cheated on my partner.
12. My simbling(s) would support me and not condemn me if I cheated on my partner.
Score:
0-2 = Low
3-5 = Medium
6-9 = High
10-12 = Danger