You can check if a PTF is applied on an IBM i machine. There are various ways to check it.

1. There is a GO PTF command that you can use to work with the PTF-related things. You need to type it on the command line and press Enter. You can check for specific PTF using option 5 display Program temporary fixes (PTF) information. Option 5 triggers the DSPPTF command and displays the command screen where you have to provide more details to be able to find information about the PTF.
2. If you have a PTF number available with you then you can use this SQL query to check if PTF is applied on your IBM i machine or not.

   Select ptf_identifier, ptf_loaded_status 
   from qsys2/ptf_info 
   where ptf_identifier = 'SI71091'

   If the above query does not return any row then that means PTF is not applied. If it returns a row then we need to check for loaded status if the loaded status is PERMANENTLY APPLIED then PTF is applied on your IBM i machine and if the loaded status is SUPERSEDED then as per the IBM page that means A later PTF existed on the system, but it may not be applied. If this PTF has been superseded, display the PTF details to determine the PTF number that supersedes this PTF, and then display the PTF to determine its status. You can get the latest superseding PTF details by also selecting the column PTF_SUPERSEDED_BY_PTF from QSYS2/PTF_INFO and then you need to check Loaded Status of PTF for this superseding PTF to confirm if the PTF is applied on your IBM i machine.

   Select ptf_identifier, ptf_loaded_status,
   ptf_superseded_by_ptf 
   from qsys2/ptf_info 
   where ptf_identifier = 'SI71091'

3. You can also use api QPZRTVFX to retrieve the PTF information. However, this api returns PTF loaded status in the form of numbers ranging from 0 to 6 instead to text because statuses are translatable text instead of special values.
   0: the PTF has never been loaded.
   1: the PTF has been loaded.
   2: the PTF has been applied.
   3: the PTF has been applied permanently.
   4: the PTF has been permanently removed.
   5: the PTF is damaged. An error occurred while applying the PTF. It needs to be reloaded and applied.
   6: the PTF is superseded, meaning another PTF with more recent changes has been loaded on the system. The ptf id that has been loaded can be found in the superseded by ptf id field.