Assuming you have configured automatic channels, the objects which you can do crosscheck are:
RMAN> CROSSCHECK BACKUP;
RMAN> CROSSCHECK COPY;
RMAN> CROSSCHECK backup of database;
RMAN> CROSSCHECK backup of controlfile;
RMAN> CROSSCHECK archivelog all;
You can also start RMAN with checksyntax option and run command without actually hitting the database. This way you can check if the commands you are feeding to RMAN are correct or not.
For example :
$ rman checksyntax
Recovery Manager: Release 10.2.0.1.0 - Production on Mon Nov 7 23:38:26 2011
Copyright (c) 1982, 2005, Oracle. All rights reserved.
RMAN> backup database;
The command has no syntax errors
RMAN> backup data;
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-00558: error encountered while parsing input commands