Most attorneys would simply bill you by the hour to do so. It depends on how many domains you have and how efficiently the attorney works.
There are a couple of different ways in which you could audit your portfolio. The easiest way would be a google search - but that wouldn't be the most definitive way.
The most definitive way would be to run a commercial thompson search (or similar search). The problem with that is it could cost $500 per domain name -- not including the attorney's time.
Naturally, there are "middle ground" ways to audit a domain portfolio. It may not necessarily be worth it to run a search through the whole list. However, before you buy a high-value domain (or sell one, depending on the agreement with the buyer), you would be well-advised to get a written opinion as to what kind of exposure there may be for that particular domain and how it should or should not be used.