Your first step, the first action you can take in this situation, is one of non-action. Simply observe. Do your best to observe your child in this new relationship. Watch how he or she interacts with the partner. Listen to how they speak to one another. Listen also to how your child speaks about this new partner. You want to gather as much information as possible before making any judgments.
Next, if you get a bad feeling about your child's partner, one thing you can do is invite everyone to hang out together. This way you can also interact with the partner on your own. Get a feel for how your child's partner treats you and your family. How they initially treat you will give you some insight into how they will treat you and your family later on, as well as how they treat your son or daughter. If this phase confirms your suspicions, then you'll want to take some action.
This is perhaps the trickiest part of the process, as how you approach your child about his or her partner will determine the tone of the rest of the discussion and perhaps the rest of the time your child is dating that person. Try to find a time to talk to your child about the relationship. Preface your remarks by saying that you only wish to offer your opinion and a little advice, and that you respect your child's decisions. If you can approach the situation gently, then you can save you and your child a lot of heartache later on. You have to trust your child at this point to understand your concern, but you have to also realize that your child is an adult who can make his or her own decisions. Think of yourself as an advisor at this point.
However, should you find that your child is in danger, either mentally or physically, then you just might have to take steps to intervene on his or her behalf. This should be an action of last resort, as the consequences of your intervention can be drastic if you misjudge the situation. But, you are the parent, and ultimately, your experience in the world gives you the tools to be the most helpful, caring parent possible for your child. That's something that no professional can really lay claim too.
The question, then, is how exactly do you intervene? The simplest, perhaps less confrontational method would be to ask to speak with both your child and his or her romantic partner. This method requires that you take an extra step in your discussion. Simply giving advice is not an option. You might request that your child and partner go to family or relationship counseling; you could offer to help pay for it as encouragement. You could offer to even join them. However, if this intervention does not work, then you may need to seek help from the justice system. Unfortunately, you cannot take out a restraining order on a third party, so you will need to talk to your child about this option. Ultimately, if your child resists, your best bet is to encourage him or her to be careful in the relationship. Be as supportive as possible, but understand that your child is an adult who can act on his or her own.