When you're checking long centric, it's a postural closure, a very gentle closure from postural position where the jaw might posture slightly forward out of centric relation.
You can check with ribbon.
You're going to put a ribbon in between the anterior teeth and just have the patient tap-tap without any hands on the jaw. If the red mark comes on an incline, then you put the patient back in centric and mark with a black mark. If the red mark on an incline is in front of the black mark, that's an indication that they need long centric. But remember, that's not anymore than just about a half a millimeter. It's a very, very small amount. And all you're going to do then is really move that surface a little forward so they don't wedge into that incline when the patient closes.
The other way you want to check is with fremitus.
Put your fingernail on each individual tooth, have the patient tap, tap, tap. If any tooth is moving, that's a pretty good indication that they are wedging into an incline and they need to have that cleared. But, let me also remind you that you cannot check this until after you've cleared all the centric interferences away from the posterior teeth. You have to get your centric relation before you can get long centric. About half of your patients don't need long centric. They'll close into centric relation, whether they're closing firmly or gently, whether they're lying down or standing up.
A common misconception
There's another great misconception that it's related to a slide from centric. It has nothing to do with that. A patient may have a very long slide from CR to MIP and have no need for long centric. It's totally unrelated to whether there is a slide or the length of the slide. It's a separate thing.