An easy way to test if a Tiel has been hand-raised is to see if it will perch on your finger.
If it does, then you are on your way to selecting a very friendly bird who will be your companion for up to 22 years.
Cockatiels are actually quite trainable, although their learning process takes a few months. They also usually take a few weeks to feel comfortable in their new surroundings, and should never be made to do what they feel uncomfortable doing.
Training sessions should be done in a quite area and the duration should be no more than three 10min sessions per day.
Food rewards, that you have found your cockatiel especially likes, can be helpful; but ensure that they are small pieces, and can be either seeds, veggies, fruits or meat (in very small quantities).
However, there are some fruits and vegetables that are toxic to cockatiels including avocado, cabbage, chestnut, eggplant, elderberry, mushrooms, onion, peach, rhubarb, green tomatoes, as well as some fruits seeds.
On the other hand, some birds will respond equally well with praise and a head scratch, which many adore.