Regulations change frequently — often more frequently than statutes. And it is, of course, critically important to base your advice to a client on the latest version of a regulation.
This page lists several good options for checking to see if you have the current rules on your topic.
The best way to check to see if you have the current regulation may be the "eCFR." While the eCFR is not official, it is very reliable.
Changes to a regulation — at least in the form of a link to the Federal Register provisions notifying of the change — will usually appear in the eCFR within a few business days.
Both Lexis+ and Westlaw Precision do their best to incorporate (or at least link to) changes in their versions of the CFR within a matter of days.
The List of Sections Affected (LSA) shows which CFR sections have been affected by final or proposed regulations. It's organized by title and section of the CFR. The LSA itself is further updated by the Federal Register. The LSA is available online (as PDF files) and in print.