How to verify a company/supplier from Germany

There is no such system - it is purely "caveat emptor" (buyer beware).
But of course you should do your due diligence and verify as much about the supplier as possible.
If you give more information, I can try to help you further.

The obvious first step is to look in the internet. What do they show on their website, how long have they been around, what recommendations or criticisms have people posted about them?... Theoretically, you could even contact some of their customers, if listed, and ask how satisfied they are...

I get spam all of the time by people claiming to be companies, say in China, that can supply something or another. Why would a company selling car parts contact me since I have nothing to do with such things? Such unsolicited offers are ALWAYS a scam.

And some scam artists could also claim to be a legitimate company. Make sure you are dealing with the right people. If someone claims to be from company xx then contact the company over their website contact. If such a company has a website then one can expect that the email address of any legitimate correspondence will be from that company; example from mail(at)companyxx.com. If it is from another site or say a gmail account then it is 99.9% chance it is a scam. Such scams are often done on the pretext of being suppliers but also offering jobs.

Another rule of thumb is that any offer of supplies or jobs that sound too good to be true are usually fake.

But like Beppi mentioned, there is no system or agency that guarantees anything. There are companies in Germany that cheat their customers in some way. Even when complaints are made, it takes years if anything happens at all. Disclosure, the whole legal system in Germany is very very slow. In the meantime, the criminals just change the name of the business and continue on as before. Such fraud can go on indefinitely.

There is an international consumer registration organization (link below), also in Germany, where one can register complaints about business scams. But looking at their website, I only see how people report, not that one can get information if a company is listed there. This might have to do with privacy, since such claims are accusations rather than court decisions saying a company is guilty. Without a lawyer it would probably be hard to even find out if there is an ongoing criminal investigation of a company. But if so, it would probably be in the jurisdiction of where the business is registered.

https://www.econsumer.gov/en/FileAComplaint#crnt