As noted in a comment under another answer, specific demarcations is a big debate. However, there is one sebset of science fiction that is pretty easy to wrap your brain around, which I will call 'assertion science fiction'.
A lot of science fiction starts with a 'normal' understanding of the real world, and then asserts a single change(*). Then, the plot of the story revolves around the interesting impacts of that change, using these to reveal or discuss our actual world.
For example, the movie 'Children of Men' posits that people simply stop having babies, and then goes on to look at the societal and personal impacts of this change. Most of these hew closely to real-world issues of nationalism, assisted suicide, the personal relationship with death and birth, and so on. The BBC series 'Black Mirror' has a series of very dark assertions like this, each it's own self-contained episode.
The underlying point here is that the change needn't be sweeping: 'Eternal Sunshine of the Spotless Mind' is an example of a science fiction movie that is, except for it's element of a memory-erasure widget, entirely the same as the real world. 'Fat Farm' is a short story about Orson Scott Card that posits the ability to mind-tape a person into a new, cloned body. It doesn't even examine all the changes such a technology would have, just a very narrow set of them. This is the heart of much of science fiction, because it gets you thinking about people inside different technological contexts. Arguably, 'The Truman Show' is science fiction but posits no 'new' technology or science, just takes our current technology and behavior to an extreme.
That combination of technology and behavior is key: to make a story you have to have compelling characters with arcs the reader is interested in, and so science fiction finds a home in examining how behavior changes - and doesn't change - given different technological contexts. These contexts needn't be huge, but the best ones drive right at the heart of what it means to be human. Gibson's 'Neuromancer' work functions at this level because it takes the concept of body dismorphia - the kludgey gluing of man to machine - and imagines a world where that is the norm. But body dismorphia is a thing people experience without going to the extreme of computers in their brain or razor-claws in their hands, so it helps us to think about our own world.
Obviously, there is a lot of science fiction that doesn't strictly fall into this form of easy categorization(**), but if you're looking for a handle to differentiate science fiction from fantasy, it is an easy one to grasp: the same roots of human behavior in a different technological context. Understanding the technology isn't strictly necessary, though it helps make it convincing because you can extrapolate more easily to the set of behaviors and interactions such a technology might impact.
(*) Ok, most science fiction asserts a lot of different changes, but it's easier to explain if we are positing a single change.
(**) I do personally think most good science fiction does have a 'pivot' technology or technologies, though: the ansible in Ender's Game, the Force in Star Wars, time traveling sentient killer robots in Terminator, the alien in Aliens, jump nexuses in Forever War, nanobots in Diamond Age, prescient police officers in Minority Report, the time machine in The Time Machine, etc. Pivot technologies are something to look for when examining science fiction because it helps us understand the context.