In an ideal world you should test each Photoshop plugin that you install in GIMP to ensure it works as expected. You should be aware that some Photoshop plugins may not function correctly in GIMP.
To try out your new plugins, go to the Filters menu and look for your plugins – they will normally be near the bottom of the menu. In the image, you can see the Flaming Pear menu item and by hovering on that the full list of filter effects is displayed. Clicking on a filter should apply that effect to the image if all is working correctly.