If you can't find out who to address your cover letter to, you should do some sleuthing – on LinkedIn, or the company website – to find the name of the department head, and address the letter to him or her. If you cannot locate the right name, "Dear Hiring Manager" is always a safe bet.
Another option to consider is calling the company and asking for the name of the hiring manager. If you have no luck with researching the name via LinkedIn and the company website, and no luck with calling the company, and you want to skip using "Dear Hiring Manager," there is another option. Let's say you are applying for a job in the graphic design department of a company. In this case, you could address your cover letter this way: "Dear Graphic Design Department Hiring Manager."
If find and use a specific name, make sure you correctly spell the name. If you have been working with a recruiter – or have the name of the company's recruiter – don’t use their name. Many recruiters don’t read cover letters, and they are not the intended target of your letter. You have a much better chance of getting a cover letter read if you send it to the hiring manager.
Since a cover letter is a formal document, you need to address it to a specific person or entity and include a related greeting. While many job postings list who to address the letter to, some leave this information out. If you can't find out who to address your cover letter to, start by doing a thorough search of the company online to identify their hiring manager. If you aren't able to locate the hiring manager, or you aren't fully confident in who this is, go ahead and address your cover letter to the head of the department. Even if you don't get it right, no one is going to hold this against you during the hiring process. If you still can't find information on the hiring manager or head of the department, write a generic greeting to the person who may end up reading the letter. For instance, you may say "Dear Research Manager Hiring Committee" or "Dear Senior Data Manager Hiring Manager." Whatever you say, pick a phrase that shows you put some thought into who may be the first one to read your letter.