consulting engineer - professional indemnity insurance?

i've employed a consulting engineer and want to check that he has professional indemnity insurance. is there someplace i can check or do i have to take his word on it?

what does this insurance guarantee me when he underwrites work for me?

You will have to ask him for a copy. It guarantees you that if he is negligent you can sue him for up to the limit on the policy.

He will have a copy of a cert for his PI, detailing his insurer, claim free year and the limit of his PI cover.
It's common practice to give a copy to any client that requests it and there should be no trouble with that. Just ask.