In the US, provisional patent applications are kept as confidential documents, away from public availability. There is no publically accessible database in which you can search or verify provisional patent filings. This is to protect and preserve your rights while your product is still in the beta/development phase. It also allows you to let the provisional patent expire without having lost your patent rights since the subject matter of the provisional was never published.
If investors ask for proof, simply show them the records of your provisional application, which includes a filing receipt. Be sure that the disclosure is done under a Non-Disclosure Agreement. About 2-4 weeks after you file your provisional patent application, you will receive an official notice from the USPTO. You can also show this Official Notice to investors.
In the mean-time, you need to have virtual patent marking putting the public on notice that you are patent pending and, if you are selling your product, in your product packaging materials, you should reference your virtual patent marking that you should place on your website immediately. If cost is an issue, please let us know and we will give you this product at a discounted rate.
When you file your US non-provisional, you will reference your US provisional Patent Application No. to claim priority. During the filing, your attorney will submit a request for the USPTO to send a copy of the provisional patent application to the International Bureau.
If you are interested in more detail related to your situation it is best to speak with an attorney.
Yuri Eliezer heads the intellectual property practice group at Founders Legal. As an entrepreneur who saw the importance of early-stage patent protection, Yuri founded SmartUp®. Clients he has served include Microsoft, Cisco, Cox, AT&T, General Electric, the Georgia Institute of Technology, and Coca-Cola.