As has been mentioned in the comments there's many ways of sensing, each with its own pros and cons.
Sensing the presence of something is actually twofold. First you need to know if there is something there, and secondly you need to try and identify what it is that's there and if it fits your criteria.
With passive sensors of the type used by Arduinos it all comes down to a matter of probability.
If you have force sensors, you know when something is there, but you can't work out what is there, only that it weighs a certain amount.
Using capacitative sensors you can get an idea about what the something is made of - how much does it affect the capacitance, etc.
Using ultrasound and/or infra-red beams etc you can get an idea of the kind of size of the object that is in the chair.
So combining them all together you can work out the likelihood that it is a human if:
- It weighs enough to be a human
- It has a suitable dielectric response to be a human
- It is big enough to be a human
Of course, there's always the chance that it's just a big dog, or a sack of potatoes.
So it's all a case of improving that probability by adding more sensors of different types, until you are almost certain that it is a human. You will never be 100% certain, of course.