I do not own a felder product but how you ensure your blade is square is typically done by using a square or digital angle finder. I will explain how to do it with a digital angle finder. Set the digital angle on the table of the saw. Turn it on and zero it out. Then raise your blade all the way up with power off (safety first). Now take the digital angle finder and set it on the side of the blade avoiding the teeth (carbide) you should now see a reading of 90 degrees or the current degree.
You should be able to use the tilt adjustment to dial the saw to 90 if it’s out of 90 degrees.
Felder should have a manual and you should be able to find literature to advise how you can adjust the tilt, square the saw etc. but that’s how majority of people I know do this. If you do use a square ensure the carbides or teeth are not making contact with the square it should be set between the teeth to check square. But using a digital angle finder is by far the easier method. Hope this advice helps.
YouTube has some great examples as well and maybe even one for your Hammer saw.