Hello,
I have been with the company I am with for 6 years now and have recently been recommended for a position at another company by a friend. I have been trying to get promoted at my current company for over year now. The comments I receive back from the hiring managers are "You have the skills and interview well but as long as someone else is applying already has the same title as the position we can't promote you." I definitely am a top performer and am doing many responsibilities of a higher level position.
I have been looking primarily within my company for a promotion but I am currently interviewing for an opportunity with another company that is a lateral move. The company is ranked third in it's industry as a "best place to work" by Glassdoor.com but when I look at the comments on that website from employees at the location where I will work they are not as positive. The company is known as providing top pay and benefits but I have heard some comments that they don't always promote from within either. My friend who recommended me felt the atmosphere was great at the site I am interviewing for but does not work for the company.
I don't want to take a lateral move (even at substantially higher pay) just to be told again that they won't promote from within. What process and resources should someone use when evaluating a company they are thinking about working for?

Can you clear something up for me?
I don't understand what you are interviewing for with your own company. If you are told you interview well and have the skills, it appears they are interviewing you for something.
Also, you cannot research promotional opportunities. The boss you might work for tomorrow might be gone in six months. These things are not static. The fact that this company might bring you in at a salary considerably more than you are currently making is a clue they like sourcing talent externally. Why would you think it stops at your pay grade? That does not mean you can't get promoted or won't get promoted.
Look at the company's last few annual reports. See if it profiles the management team. Read the exec bios. See if there is a trend of them being parachuted in from other companies. And stop worrying about things you can't control.
My Five Metrics for Evaulating a Job Offer
I've found that it can be hard to evaluate a job offer. And the interview process is better if I have some structure to ask questions.
So here are five key groups that I use to evaluate companys/job offer:
The Five Metrics