After you sync custom objects down to Marketo from Salesforce, you need to know where to view the field data in Marketo.
Marketo does not display the data from the fields on the Salesforce Custom object in the lead record. The best place to view Salesforce Custom Objects and records would be in Salesforce.
The best that can be done is to create Smart Lists in Marketo with filters of "Has [CustomObject]" and add a field constraint with the specific value that you are querying for. This will bring up a list of leads with that Object and field data.