Suspension bushings typically need to be replaced about every 2 years depending on use and should be inspected yearly at a minimum.
Usually one can "shake down" the suspension system while the trailer is in the air and tires off during an annual maintenance. This process is done by grabbing ahold underneath the movement points and looking for play while it is being rocked in an up and down motion. Another way to tell is a visual look around the bolt and nut heads on the suspension links to find if you see any of the hole that the bolts are going thru. We usually refrain from recommending this be the standard inspection due to if you do see the hole it means your system is not far off from breaking thru.
The leaf pack should be inspected as well for fatigue/sagging which can lead to breakage while under load. The u bolts should always be replaced when replacing leaf springs as they are designed to stretch.
You can always call our service department to inquire about getting an annual maintenance and inspection on your trailer. 253-922-0771