The best way to tell if a horse is traveling is just to tune into what the horse is doing underneath you.  When warming up, at the walk pick up the reins, make sure the neck is straight, that the shoulders are traveling straight (not floating back to the rail if your on the inside track etc. etc. etc.).  Riding the inside track will give you a chance to work on your horse's straightness in the warm up and throughout your ride.  It allows you to feel better if the horse tends to fall to the outside or the inside.  A great addition to the inside track is if mirrors are at each end of the ring so that you can see as well as feel if your horse is straight.

On the circle or through corners the horse should travel so that the inside hind reaches forward and under the horse an steps in the same line as the inside fore.  This can only be truly done if the horse is traveling straight before the circle and if the outside shoulder and hip are not being dropped to the outside.
 

The horse needs to not only be traveling straight in his path, but his spine must be straight as well.  The horses neck should be straight out of the shoulders, his back straight his hips in line with his shoulders.  On the Curve the bend should be even from head to tail.  Training to achieve proper straightness helps lead to balance, suppleness, lateral work, straight even jumping form with no tilting or twisting over the fence.