The DX5e has a long audible beep for center trim, and for the left & right trim limits. If you try the rudder trim for example, if you press and hold it left (from center trim) you'll hear several beeps going higher in tone until it stops at a longer high tone beep. This is your far left trim limit. If you release the trim button and push it left again, you will hear no more beeps since you are at the left limit. Next push and hold the rudder trim right, and you'll hear several beeps going down in tone until it stops at longer beep. This is your center trim point. If you release the trim button and push it right again, you'll hear several beeps going lower in tone until it stops at a longer low tone beep. This is your far right trim limit. If you release the trim button and push it right again, you'll hear no more beeps since you are already at the right limit.

Once you try this a few times, the beep tone feedback makes it pretty easy to figure out when you're at center trim.