Before you go any farther, if you think you have running form issues, spend the money to get some qualified help. You might be surprised of the benefits of investing in as little as 1 or 2 sessions with a good running coach.
So, with that out of the way, here are some thoughts from my personal experience.
If you ever paid attention to your fellow competitors at a running race, you may have noticed that just about everyone seems to have their own running style. Some are graceful and others anything but (myself included in this group), however, there are a lot of runners with less than perfect form who are running moderately fast and long, so, I don't think performance provides a direct correlation to form. From my personal experience, here were some leading indicators.
Getting Injured Often - This seems obvious enough on the surface, but, where the difficulty comes in is defining "injured". Maybe you aren't getting full blown calf strains, but, maybe you aren't recovering very well or the thought of running two days in a row seems near impossible. These more subtle signs may point to form issues with the big injury lurking somewhere in the future unless you get things under control.
Your Running Is Loud - It is easist to detect this when running on a treadmill. If your running produces a loud, thud, thud, thud it is likely that you are bouncing or overstriding. This not only contributes to injury, it is plain inefficient. When you overstride (feet landing in front of hips) you are stopping some of your forward momentum with every step. An efficient stride will land softly under the hips with most of your energy used to propell your forward.
You Have A Hard Time Incresing Distance
Your Running Is Loud
Your Head Bounces Up and Down Exessively
Your Feet Are Landing In Front Of Your Hips
One HUGE word of caution on trying to change running form. Don't Try To Change Overnight! If you have been running for a while, your entire body has adapted to your current form, good or bad. Any change has to be worked in gradually so as not to cause injury rather than prevent it.
Hire A Running Coach - Most health clubs have multiple trainers on staff who can help you with your running form and will probably be very affordable for a few sessions. They will be able to point out your inefficiencies quickly and give you a plan to improve your form that you can then work on independently. You may find that by correcting one or two major inefficiencies your training moves to a whole new level.
Learn What Good Form Looks Like
Run In Front Of A Mirror - If you can't find or afford a coach, the next best thing is to be your own. Leave the headphones at home, forget about the tv programs on all the flat screens and find a treadmill in front of a mirror. Spend an entire session watching your form in the mirror. If someting seems out of whack make a minor adjustment and evaluate if it looks better and more importantly feels better.
Slow Down.... Sometimes
Don't Worry About Perfection - Every body is unique and what works for me might not work for you. The trick is to keep trying to improve and keeping what works for you and what doesn't. Your body should let you know what works by rewarding you with better recovery or more comfortable and enjoyable training sessions.
Have Patience
Warm Up.... Always
Cool Down.... Always
Roll With It
Stretch It Out
Consistency Is King
Run Support Exercises