How to use the VALIDATE options in RMAN - both 10g and 11g

Validate if database can be backed up (10g)
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BACKUP VALIDATE validate that all database files and archived logs can be backed up.
When you run BACKUP VALIDATE, RMAN reads the files to be backed up in their entirety, as it would during
a real backup. RMAN does not, however, actually produce any backup sets or image copies.

Validate against logical corruption (10g)
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BACKUP VALIDATE CHECK LOGICAL checks for logical corruptions.
In a logical corruption, the contents of the block are logically inconsistent.
Examples of logical corruption include corruption of a row piece or index entry.
If RMAN detects logical corruption, then it logs the block in the alert log and server session trace file.

Validate if a file or backupset can be restored (10g)
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RESTORE ... VALIDATE test whether RMAN can restore a specific file or set of files from a backup.
RMAN chooses which backups to use.
Validation of backups of the datafiles only reads the backups and does not affect the production datafiles.
RMAN reads all blocks in the backup piece or image copy. RMAN also validates offsite backups.
The validation is identical to a real restore operation except that RMAN does not write output files.
The lack of error messages means that RMAN had confirmed that it can use these backups successfully
during a real restore and recovery.

Validate against physical corruption (11g)
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In Oracle Database 11g, a new command in RMAN, VALIDATE DATABASE, makes the operation of checking that the database is healthy and has no bad blocks trivial.
It can substitute the dbverify tool used in previous releases.
If physical corruption is detected, it logs into the Automatic Diagnostic Repository.
RMAN then produces an output that is partially shown below:

RMAN> validate database;

 You can also validate a specific tablespace: 

 RMAN> validate tablespace users;

 Or, datafile: 

 RMAN> validate datafile 1;

 Or, even a block in a datafile: 

 RMAN> validate datafile 4 block 56;

Add all the archivelogs, too:

RMAN> validate database plus archivelog check logical;

The VALIDATE command extends much beyond datafiles however. You can validate spfile, controlfilecopy, recovery files, Flash Recovery Area, and so on.

Oracle 19c documentation here