Search the web for manufacturers in the area. Their web presence, as superficial as it may seem, will speak to their concern with presenting themselves favorably to their potential customers. It's just like going into an interview. Having a suit and tie, and being well groomed does not necessarily mean that you can do the job, but not doing those things will surely indicate that the deal is off. 

Once on their site. Look for information that let's you know that they know what they're talking about. Do they provide information on their services? Do they have resources for customers visiting their site? This kind of communication shows that they care about educating their customers about the things they care about.

Now it's time to reach out. Connect with them over the phone and ask questions that are relevant to what you need. Don't be shy about asking exactly what you need from your suppliers. Can you hold product? What is your scrap rate? How is your inventory system organized? How many customers do you currently have? How do your prices compare to your competitors? etc.

Schedule a meeting at their facility. This takes time out of your day, but it's worth the leg work on the front end. Tour their facilities with the team. Ask questions and observe processes specifically regarding program management. How they run the show from the top down speaks volumes to whether you will see your product on point and on time.

The results matter. You've established that they know what they're talking about, but can they deliver on that? It's worth taking some time to ask them about what kind of quality and punctuality can be expected and how confident the team is at delivering those. Ask how they currently ensure that they are meeting current customer requirements and if they would be willing to possibly putting some fees at risk to prove it.

Ask them for recommendations, based on the information that you've provided them, on ways that they can tailor their services to meet your specific requirements. Will they hold product? Can they customize their quality requirements to meet your product needs? The proposed package should include all of your requirements. Their willingness to accommodate your needs speaks volumes about a supplier's viability. 

Contrary to popular opinion, references given by potential suppliers don't really mean that much. Companies these days always have a few references in their back pocket. This, however, doesn't really give a complete picture of the company's ability to consistently deliver. You're looking for consistency, and the real way to do that is to look at the big picture.