I have of course no idea if you unserstand German, but here are two homepages explaining what Steuernummer and UST numbers are:

http://www.bff-online.de/ust/useg/UST_IdNr_.html
http://www.lrz-muenchen.de/~rgerling...uernummer.html

If you need more help, don't hesitate to send me a PM. You will understand that without the number or the name of the firm I cannot check the bona fide of the company you are referring to.

These two numbers are NOT a guarantee that the company is financially stable, it is just the account number that each firm has for the collection of VAT and other taxes.

There are however ways to find out, but one would need more details.

Good luck