Unfortunately it is difficult. Refer to this older post from someone at MS who has a recommended solution. A partner # means nothing really, in my opinion. I can go get a partner # of someone who is a partner and claim it as my own. My recommendation is to use the Find a Partner site listed in the link to search for the vendor you are considering using. Ensure that the vendor that is listed on the partner site lines up with who you're actually talking to.
I think if you have any concerns after searching then maybe you should consider a different vendor. Alternatively contact MS and ask them about the vendor in question.