You can check them visually without removing the rockers. Just compare the position of the seals. If one -or more- have popped off, it should be apparent.
Getting down to the seal to remove it requires supporting the valves with compressed air so they don't fall into the cylinder when the keepers are removed. If you only do the intakes, you might get by without the air, but the piston will need to be at exactly the top. I have always used compressed air because I have always changed both intake and exhaust seals.