Is there a way to determine if the implants are slowly disintegrating?

The implant material doesn't really disintegrate, but there is a good chance that given the age of your implants, there is likely a leak in one or both of them. When this happens you may not notice it. One way to make sure is to have an MRI to look. If you are happy with your breasts otherwise this is what I would recommend. If you are no longer happy with your breasts, you could just get the implants exchanged for new ones and forego the cost of the MRI.

Polyurethane silicone implants were withdrawn from the US market in 1991 due to safety concerns. Polyurethane can degrade into 2,4- and 2,6-toluene-diisocyanate diamines (TDAs), which have been shown to cause cancer in laboratory animals.  Given that your implant are 28 years old, the likelihood that one or both has ruptured is quite significant.  I recommend MRI evaluation of the breasts.  

I doubt that your implants are disintegrating.  However an MRI should be able to detect the presence of the wall of your breast implants.  Please check with your plastic surgeon or primary care physician to request an MRI.