RMAN validate incremental level 1 backups?
First, you need to discover which backup is your level 0 and your 1. Then you need to validate the level 0 and the level 1 backups, independently.

 LIST BACKUP SUMMARY;


From the list, find the key for your level 0 and your level 1 backups. 
Then validate them via:

 VALIDATE BACKUPSET 370, 412;


Substitute your key values in that. You can also add in the key values for archive log backups, if any.

See 
https://docs.oracle.com/cd/B28359_01/backup.111/b28273/rcmsynta053.htm#RCMRF162 Example 2-157 Validating a Backup Set, for more details.



This is beyond the scope of your original question, but may be of benefit to you:

I run RMAN backup configuration scripts as part of my "new Oracle instance" checklist. That script looks like this:

run {
    CONFIGURE CONTROLFILE AUTOBACKUP ON;
    CONFIGURE RETENTION POLICY TO RECOVERY WINDOW OF 14 DAYS;
    configure compression algorithm 'MEDIUM' optimize for load false;
    configure device type disk backup type to compressed backupset;
    configure device type disk parallelism 2;  }


Then I do my backups via this, with either a level = 0 or a level = 1, depending on the day of the week:

backup device type disk incremental level = 0 section size 1g database plus archivelog tag 'level 0';
backup current controlfile tag='Controlfile Level 0';
backup spfile tag='SPFile Level 0';


I then do the following. Probably not necessary, but...

allocate channel for maintenance type disk;
backup validate check logical database archivelog all;

crosscheck archivelog all;
crosscheck backupset ;
crosscheck backup device type disk;
release channel;