The hips do not turn level in the golf swing. Good players have their hips tilt in the backswing. The reason for this is as the golfer is turning the trail leg loses flex and gets longer and the lead leg gains flex and gets shorter. This causes the hips to tilt in the backswing.
To illustrate this I put an alignment rod through my front belt loops and you can see with the right leg longer then the left it creates a side tilt. Using 3D gathering systems like Gears Gears Sports or 4DMotion we see the best players with a tilt between 5-20* depending on the player and club being used. 4DMotion is a great option to get a sensor or two and get feedback from your swing.
Now the question becomes how long do the hips stay tilted in the downswing? Into lead arm parallel on the downswing the pelvis loses a few degrees but still are tilted between 8* and 15* for a mid iron. Lead arm parallel is the point that the vertical force on the lead foot spikes and the hips start to rise. We see the hips level out between lead arm parallel and shaft parallel. Here is an image from Gears showing the the level position for this player. After this the lead hip gets higher all the way to the finish.
Below are 2 images. First image on the left is where I pushed my hips forward fast which caused my head to go back and the left hip to get high. The second image on the right I was falling more into the left leg before pushing off the gourd. Was doing a drill covered HERE in this YouTube clip working on getting the alignment rod between the belt loops to contact the alignment rod in the ground behind me in the white area before impact. Left photo was a thin shot where the club landed slightly behind the ball due to tilting back and the right photo was a solid strike with ball then turf.
How can you train this? One option would be to place an alignment rod between your belt loops and at the top of the swing have it point towards the ground around 5′ in front of you. You could put an alignment rod on the ground outside the ball as a reference point. Another option is covered in the below image and in THIS YouTube clip. Using a 6-10′ string around an alignment rod or something down at the ground you can hold one side on your left hip and the other on your right. As you turn your trail hip back it will pull your lead hip forward. Can do this holding it in your hands as well and give you the feel for tilting the shoulders on top of the tilted hips.