Is there any way to detect a subject being focused or highly motivated (assuming that these two states are essentially the same) using any sort of external detection, such as fMRI or EEG? How would the accuracy of these measurements be compared? I have a hard time imagining what the "ground-truth" of these measurements might be.
Focus and motives are very different cognitive processes. Essentially focus would be attention, while motivation is a complex neurocognitive process, such as hunger or thirst, which drives attentional orientation.
I'm going to have to break this down a bit further because motivation is largely considered two processes 'wanting' and 'liking'. 'Wanting' is the drive to obtain a motivational object, such as food. While 'Liking' is the assessment of the motivational object. So if a digestive tract is empty of food it will typically send chemical signals telling the brain its empty, amongst a whole host of other factors but lets not get into that. As a result we will 'want' food, and this alters the hedonic value of food, which ultimately influences its salience (how much it grabs your attention). And its representation in the brain, all of this is done to draw your attention towards the motivational target, or make us seek it out. Once the target i.e. food, is obtained it is consumed, this is the point at which it is assessed for how much we 'like' it. All of this is done by a neural system, often referred to as the reward circuit. The reward circuit is made up of a number of brain areas (nuclei), so many it would take an dissertation to write about them all however the main constituents are: Nucleus Accumbens, Ventral Tegmented area and the parabracial nucleus. Changes in these structures appear to influence activity in the cingulate cortex (likely to do affective assessment and memory) and orbitofrontal cortex (attention, inhibition, hedonic value, etc). As you can see this is just gross anatomy, these structures are involved in a variety of cognitive processes.
Going by your question I imagine you are interested in the drive to behaviour or determination. The nucleus Accumbent shell is considered responsible for this, although I think thats like saying wembley is responsible for one of two teams winning a football match. The resolution in functional signals has a long way to go. Anyhow endocannabinoids are thought to be responsible for modulating the increase in eating or rather 'wanting' to eat, they do this by increasing the hedonic-value of the motivational target. Hence cannabis give people the 'munchies' and can make them feel sexually aroused.
So to answer your question can we measure this? Yes and no. fMRI can detect motivation-related activity in NA shell, but there is alot going on in that area so there is bond to be noise that must be controlled for in any design. But essentially you can show someone images of food before and after eating and measure their neural responses. However I must stress that the NA is not the only area involved in this motivational process.
As for EEG, research is a bit inconsistent and I would feel uneasy recommending anything from this area. This is mostly because motivational processing is largely conducted in subcortical structures, which EEG isn't really the best method of investigation for. However it can be used for detecting arousal, but cognitive arousal can occur as a result of merely interesting or treating stimuli.
Another method are implicit cognitive measures and eye tracking, this is a growing field. Measures include, visual-search tasks, dot-probe, stroop, go/n-go, attentional blink tasks etc. What this area tends to show is that people are more attentive to motivationally-relevant stimuli i.e. more attentive to food when they are hungry. Although some research shows that people attend to such stimuli regardless of motivational state, the methods here are constantly being updated as they are still relatively new methods.
Finally and importantly do any measures discriminate between levels of motivation? No, not any of the neural or cognitive measures I know of anyway, most are lucky to detect differences between fasted and fed states. There is work going on in this area, but so far differences in levels of motivational state are only collected on explicit measures such as VAS scales asking someone what there level of motivation is.
Sorry for lack of references SE only allows you two references on the first post in a new area of SE.