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This is a complicated and loaded question. As Neuroskeptic noted, our understanding of consciousness is very poor (in fact, we don't know how to define it most of the time). To see some of the best current definitions, take a look at:

What are current neuronal explanations and models of 'consciousness'?

We definitely can't infer arbitrary properties of consciousness from something like fMRI. In general, we don't have a clear mapping from brain to mind, and thus all studies are inherently correlational in nature. Since all the work is correlational, it is thus not usually mentioned and the popular press tends to misconstrue this as a for-sure causal connection. But does that mean we can't infer any thoughts? No!

We can "read thoughts" well enough to produce an brain-computer interface. Adapted from my earlier answer:

Erik Ramsey is locked-in syndrome patient and is incapable of movement apart from his eyes. However, he has control of his brain enough to be distinguished by electrodes implanted into his cerebral cortex (Guenther, 2009). Although the sensors were implanted in the motor-area responsible for speech (which you would expect typical inviduals to have control over, since they can speak), the understanding of how to decode this is not full. Thus, the patient had to train in order to adapt to the system:

Accuracy of the volunteer's vowel productions with the synthesizer improved quickly with practice, with a 25% improvement in average hit rate (from 45% to 70%) and 46% decrease in average endpoint error from the first to the last block of a three-vowel task.

In a much more dramatic experiment, Adrian Owen has used brain imaging techniques as means of communicating with vegetative-state patients. It is known that visualizing playing-tennis and walking around the house produce very distinct (to an fMRI) activation in the brain. So Owen used that capability to allow a vegetative-state patient to answer yes-or-no questions. He asked the patient to imagine playing tennis for yes, navigating the house for no (Monti et al., 2010). Hence, neuroscientists think they can distinguish the thought of walking around the house versus playing tennis.

Of course, it is hard to say if this can be continued arbitrarily far, in practice or even in theory. For more info, see this question:

Is performance reducible to brain activity in an unambiguous way?

References

Guenther, F.H., Brumberg, J.S., Wright, E.J., Nieto-Castanon, A., Tourville, J.A., Panko, M., Law, R., Siebert, S.A., Bartels, J.L., Andreasen, D.S., Ehirim, P., Mao, H., & Kennedy, P.R. (2009). A wireless brain-machine interface for real-time speech synthesis. PLoS ONE, 4(12)

Monti MM, Vanhaudenhuyse A, Coleman MR, Boly M, Pickard JD, Tshibanda L, Owen AM, & Laureys S. (2010). Willful modulation of brain activity in disorders of consciousness. New England Journal of Medicine, 362(7):579-89.