If you really go overboard it will show pretty quickly. Sometimes it can creep up on you if salts start to accumulate in your medium, usually that is less sever and comes on a tiny bit slower and really subtle on the tips. If you really burn them they will get crispy on the edges.
I always check my run-off when I see a little bit of burn, sometimes it's from salts starting to build up slightly(usually when life gets in the way and I can't make 30+gallons of ro in time to feed them for a day or so and the medium gets a little drier then I'd like)-I might be putting in say, 800ppm and see a tiny bit of burn, I know that is on the lower side so I check my run-off and it's usually way higher, like 1100-1200-so I know it's not the strength of the feed as much as the salts that have accumulated in my root zone.
So if you do experience some slight burn, check your run-off and see if your feed is too strong, or you just have salts accumulating in your medium. It's better to correct it immediately with a flush or heavy irrigation then to just lower your feed and let it correct itself. If you JUST lower your feed it doesn't really help with the accumulated salts and the problem takes longer to correct while your plant is still getting burnt.
