Ur right, I thought so too, but what i did find useful were their demeanors and facial expressions. Also in some cases, what they chose to talk about and how they said it.
Also in some cases, I've found that what someone says might reflect the opposite about their socionics type. For example, if someone says "I dont have a lot of friends," it could mean that they are actually extraverts who do have 20 friends but feel that's not enough and wish they had more friends. If someone says "I have a lot of friends" it could actually mean they are introverts with 3-4 friends and this is more than enough for them. You get my drift.