Short of getting coached directly, the best tool for the job is taking video. There are a number of digital video recorders that have at least an hour of video available on the device. When you are training by yourself, you want a video recorder that can stable enough for you to stand up and trust that it won't fall over while you are lifting. You may need a mini tripod if that's going to be difficult.
- You won't be able to get your full body in the shot, so decide what's the most critical part and position the camera so you will capture that.
- Experiment and find the best angle and distance to capture the video. You might be constrained by the environment (i.e. where equipment is placed, high traffic areas, etc.)
- Review the video after every set. It should at least tell you if you are getting deep enough on squats, etc.
After the training is done, take the video off the device and do a bit of post processing. At the very simplest, you'll want to remove all the dead time. There's even some free software that can help you analyze your movement patterns called Kinovea. There's a pretty steep learning curve with that, but you'll be able to tell if the bar is moving in a straight line, and it will help you understand where you need to improve.
I never train with a partner either, mostly due to logistics. However I've found that using video, I can see and correct things myself, and my coach who is in another state can review my technique and progress. He's been able to identify supplemental work I needed to do to enable better technique at some of my lifts.