If you use something like Workbench [1] or the Developer Console, you can just use SOQL to query the Email records.  I'd start with a query like:

select FromAddress, ParentId, Subject, TextBody from EmailMessage where ParentId = '500xxxxx'

and if that returns results then you are good to continue on with your trigger.  Your trigger will be something like:

trigger EmailToCaseReply on EmailMessage (after insert) {
     List<CaseComment> newComments = new List<CaseComment>();
     Set<String> emailAddresses = new Set<String>();

     for (EmailMessage email: trigger.new) {
          emailAddresses.add(email.FromAddress);
     }

     emailAddresses.remove(null);

     if (!emailAddresses.isEmpty()) {
          Map<String, User> userMap = new Map<String, User>();

          for (User user: [
               select Email
               from User
               where Email in :emailAddresses
          ]) {
               userMap.put(user.Email, user);
          }

          for (EmailMessage email: trigger.new) {
               if (!userMap.containsKey(email.FromAddress)) {
                    continue;
               }

               newComments.add(new CaseComment(
                    ParentId = email.ParentId,
                    CommentBody = email.TextBody,
                    IsPublished = true,
                    CreatedBy = userMap.get(email.FromAddress).Id //This line may not work
               ));
          }

          if (!newComments.isEmpty()) {
               insert newComments;
          }
     }
}

NOTE: This code has not been tested and may contain typographical or logical errors.

The CreatedBy line when creating the new comment may not work.  If it doesn't, just omit it.  You may want to adjust / omit the userMap part if you want anyone to be able to create comments on the case.  Or adjust it to look at contacts if you want to make sure they have a valid contact record.

You'll want to add additional logic to make sure you don't get stuck in an auto-reply loop, but that's a problem to look into after your initial implentation.