I had an allergic reaction to ibuprofen 25 years ago and just took 600mg by mistake? what should i look for and do in response?

Watch for the serious allergic symptoms such as shortness of breath, wheezing, lip and tongue swelling, abdominal pain, diffused skin rashes with intense itching & if those happen, go to urgent care or ER for care. Otherwise, if you have some rashes with itching or just itching without rash, you can just observe. You're safe if there is no symptom and you may not be allergic to ibuprofen at all.