The Maven Versions plugin and its display-dependency-updates
mojo are what you're looking for:
mvn versions:display-dependency-updates
Here is what the output looks like:
[INFO] ------------------------------------------------------------------------
[INFO] Building Build Helper Maven Plugin
[INFO] task-segment: [versions:display-dependency-updates]
[INFO] ------------------------------------------------------------------------
[INFO] [versions:display-dependency-updates]
[INFO]
[INFO] The following dependency updates are available:
[INFO] org.apache.maven:maven-artifact ........................ 2.0 -> 2.0.9
[INFO] org.apache.maven:maven-plugin-api ...................... 2.0 -> 2.0.9
[INFO] org.apache.maven:maven-project ....................... 2.0.2 -> 2.0.9
[INFO] org.codehaus.plexus:plexus-utils ....................... 1.1 -> 1.5.6
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 17 seconds
[INFO] Finished at: Fri Aug 15 10:46:03 IST 2008
[INFO] Final Memory: 10M/167M
[INFO] ------------------------------------------------------------------------