premise: How to replace an empty ink cartridge<br>Write down the printer brand and model number. You will need to know both in order to find the correct replacement ink cartridges. If you can't find the model number, check the documentation that came with your printer.
hypothesis: The model number will not be on the printer. 
reason: Not sure why it's difficult. The paragraph says to check the document if you have to in order to find the printer model.  That suggests that the printer having the model number on it will be true or not true. 