Salesforce Apex List Required Fields

gistfile1.cls

SObjectType sot = Your_Custom_Object__c.SObjectType;
Map<String, SObjectField> fields = sot.getDescribe().fields.getMap();
for(String k : fields.keyset()) {
	DescribeFieldResult dfr = fields.get(k).getDescribe();
    if(!dfr.isNillable()) {
		System.debug('REQUIRED FIELD: ' + k);        
    }    
}