I have found the Baseline 2.5 CanBeFrozen API call to be useful for this.  For those unfamiliar with it, "The method executes the same checks as the Freeze method without freezing the baseline or labelling the version objects.". What you get back is a report with headings like:

ConrefNotResolved

HyperlinkTargetMissing

NoVersionSelected

ObjectNotReleased

I start running those reports well ahead of an upcoming baseline freeze (we have very big baselines shared by tens of publications).

Hello Frank.

What happens now if we click Freeze baseline in Publication Manager?

Do we need to have published in order for this to correctly report any issues?

Hi Frank - let me discuss with the user group and get back to you on this one.

Hi Elizabeth, validating a baseline can only be done correctly if we publish the document at hand. Therefore, a Validate Baseline option would mean the same thing as publishing the document. 

Please take a look at the following post I made where I suggest a new approach to solve this issue: community.sdl.com/.../modify-pub-manager-so-it-does-not-try-and-resolve-xrefs-that-are-conditionalized-out-of-the-pub