public static Map<String,String> getCustomFields(String objName){
    Map<String,String> customFields = new Map<String,String>();
    SObjectType accountType = Schema.getGlobalDescribe().get(objName);
    Map<String,Schema.SObjectField> mfields = accountType.getDescribe().fields.getMap();
    for(String fld : mfields.keySet()){
        if(fld.contains('__c'))
            customFields.put(fld, mfields.get(fld).getDescribe().getName());
    }
    System.debug('customFields => '+customFields);
    return customFields;
}

poc.getCustomFields('Account'); // to get account custom fields

public static List<String> getCustomFields(String apiName) {
    SObjectType objType = ((SObject) (Type.forName('Schema.' + apiName).newInstance())).getSObjectType();
    Map<String, SObjectField> fieldsByName = objType.getDescribe().fields.getMap();
    List<String> customFields = new List<String>();

    for (SObjectField field : fieldsByName.values()) {
        if (field.isCustom()) {
            customFields.add(field.getDescribe().getName());
        }
    }

    System.debug('customFields => ' + customFields);

    return customFields;
}