How to tell if i am underweight and how can i gain weight quickly?

No way: It has to be gradual. You can try ensure or boost, but it is quite expensive. Try juices.

Gaining Weight: Consider using a BMI calculator. Eat healthy foods that are high in calories. Eat 6 meals /day. Work on strength training to build large muscle groups. Work out w heavier weights w fewer repetitions. Sleep at least 8 to 9 hrs/night. Hydrate well w water. Foods to help w healthy weight gain: almonds, cashews, pecans, sunflower seeds, flax seeds, walnuts, hummus, avocados, granola & trail mix.