Gridlock. We understand this word in terms of traffic, but the same meaning can be applied in relationships. When you have come to a point where neither of you will budge on an issue, waiting it out probably won't change anything. You will remain stuck and it'll actually create a more hostile environment for resentment to grow.
Wandering hearts. When you're at a point where you are seriously thinking "I can do better", "why do I put up with this", or "I don't need this in my life", then you are getting closer to throwing in the towel and further from working things out with your partner. It becomes a slippery slope if you should cross this line, so it's a good sign to get help.
Living in Re-runs. Having the same argument over and over again is not only frustrating, but also feels hopeless. And eventually this argument and these feelings can cross over to other issues and start leading you to feel frustrated and hopeless in other areas of your relationship. It then can consume other good feelings you might have in the relationship, which makes it more than enough of a reason to get help.
Walking on eggshells. If you or your partner feels so emotional about a topic that you can no longer discuss it without an emotional escalation or breakdown, or if you feel like you have to avoid certain words in order to keep the peace, then it's a huge red flag to get help. The longer this is allowed to go on, the thicker the layers of problem there will be.
Sometimes these signs come in pairs, threes, or solo. Whichever the case, I would encourage addressing it while the problem is still young and less evolved. Relationship problems that don't go away will continue to haunt the relationship until an honest solution is reached. It will be risky to seek help because the answer you're looking for may not be what you find. However, it can provide much needed relief.