I'm sure many people have already said the same thing on here but I figured I'd throw in my 2 cents...
Here's some things that might help you out.
First off, research, there are hundreds of websites out there now Salary.com etc. as well as any number of professional organizations that anually poll thier members' salaries to get an idea of the market conditions.
Secondly demand, you don't have to go directly looking for other jobs to know whether or not your field is in demand. I know that my profession (Fire Protection Engineering) continues to have a high demand despite the rough economy right now because there are so few of us. If you are a civil however things might be a little lighter in terms of demand due to the recession. Your ability to relocate will help in this situation because it removes the employer argument, "yes there is demand but if you stay with us you don't have to move."
The third thing is value, yes you may be doing the same job you were doing before you got your PE, but can the company charge a client more money for your services now because of your certification? (I'm betting most of them can.)
Commonality, (kinda parallels demand) are there other people in the company that can do your job or are you the only one? Even if your a civil, if your the only civil in the company they might be more inclined to pay keep you than to go outside and go after an unknown quantity just to save a few bucks.
Finally there are of course the intangibles, do you have other skills so they can plug you into something else if your field falls slack? What is your relationship with the company? What have your past work habits been like? What is your potential?
I will say though whatever you are looking for don't be an *ss about it...
That's were the research comes into play make sure you KNOW what the economy holds I would never discourage anyone from going out and trying to make the best they can out of thier situation. If you think you can get better from your company. Pursue it...respectfully...but go for it, I honestly think the worst that can happen if you go about it the RIGHT way is they could say, "well, we'd like to give you a raise but we just can't afford it right now." If they become jerks about it themselves maybe you need to reevaluate were you are in life and if they fit into your long term plans, be smart but don't be a doormat either.
One other piece of advice. If you decide to go looking elsewhere in an attempt to leverage your company for more money (and I don't necessarily recommend this but if you do). DON'T BLUFF.... make sure you have a genuine offer and be prepared to take the other job if your going to go down that road and make sure you leave amicably.
Hope this helps...