I am the legal guardian of my disabled 26 year old son. I want to check his credit report to make sure noone is using his identity. how can I do this?
In the US, individuals are entitled by law to a copy of their report from each of the major bureaus via annualcreditreport.com. You can get a copy from each bureau once every twelve months. If you use this service and stagger each bureau, that's effectively a check every few months, which should be plenty.
The FTC maintains info about this program on their website.
There are also free services that allow more frequent access, such as creditkarma.com. The formula used for calculating the scores will be different for these sites versus that which is used for lending, but that's not typically an issue if you are more concerned about fraud than lending.
In Germany, send a request for a copy of all personal information they have about your ward to the SCHUFA. They are required by §15 DSGVO to give you one free credit report per year that way. You can find the form here. If you still need a regular report for some reason, they can be ordered here for a fee of 29,99 €.
When you are requesting the credit report not for yourself but for your ward, you are theoretically required to also send them a scan of your certificate of guardianship ("Betreuerausweis") which you received from the "Amtsgericht" when you agreed to take on the guardianship of an adult. This proves that you are the legal representative of your ward. However, I am pretty sure that if you simply fill out the form on the SCHUFA website with the personal information of your ward and scan their personal ID ("Personalausweis"), they are going to send it to him because they likely have no way to know that your adult son is disabled and has a legal guardian.