[SalesForce] How to make a inputFile field required

You could do the formatting in VF-only, but it will also show while not in edit-mode:

 <apex:outputLabel value="picklistLabel"/>
 <apex:outputPanel layout="block" styleClass="requiredInput">
     <apex:outputpanel layout="block" styleClass="requiredBlock"/> 
     <apex:outputField value="{!MyObject__c.MyPicklist__c}" >
        <apex:inlineEditSupport event="ondblclick"/>
     </apex:outputField>
</apex:outputPanel>

Note that you should still do the Validation with a validation rule, as this is merely a layout change.

There is no generic method but you could create your own without having to hardcode and check the fields individually. For this, you need to do a describe on the object and include that in your controller or in a (before or after insert/update) trigger:

   List<String> requiredFields = new List<String>();    
   Schema.DescribeSObjectResult r = User.sObjectType.getDescribe();
    Map<String,Schema.SObjectField> M = r.fields.getMap();
    for(String fieldName : M.keySet())
    { 
        Schema.SObjectField field = M.get(fieldName);
        Schema.DescribeFieldResult F = field.getDescribe();
       // if F.isNillable() is false then field is mandatory
      if( F.isNillable())
         requiredFields.add(fieldName) ;
   }

   for(YourObject__c yo : listOfYourObjectRecordsToCheck){
      for(String s : requiredFields){
         if(yo.get(s) == null){
            s.addError('A required field is missing'); // or any other way of raising an error
            // we can immediately continue to the next record
            continue;
         }
      } 
   }