There's no strict delineation, and no way to do it even if you wanted to. Even a moderately broad definition of history ('stuff that happened to people in the past') would inherently overlap with a huge number of other humanities and social sciences, which rely on data and evidence from past events to do their work. If you take a really broad definition of history, it's possible to argue that all other disciplines are just varying forms of applied historical studies, since for data to be accessible it has to have already been generated and collected.

In turn though, historians borrow methods and are influenced by ideas from many other disciplines. These processes are very changeable and affected by discipline-wide fads and trends. In the 1960s and 1970s, for instance, social history was the coming thing and many historians embraced social scientific methods (eg from economics) not just to open up new perspectives on the questions that drove social historians (eg how ordinary people lived their lives) but also to lend their work a seemingly more empirical basis in an intellectual context where quantification meant legitimacy. By the 1980s and 1990s, the pendulum had swung back towards literary and anthropological influences, as historians became more interested in culture and embraced the inherent subjectivities to historical work (along the way, exposing some of the underlying problems with a previous generation's seemingly unimpeachably empirical and verifiable work). Today, to take just a couple of examples, digital historians adopt a broad range of methods from computer sciences to try and gain new forms of insight into large datasets or network analysis, while history of emotions scholars draw on psychology and neuroscience to try to understand how humans felt things in the past.

However, it's probably safe to say that while historians have a tendency to be methodological magpies and steal bits and pieces from other disciplines that are concerned with overlapping issues and questions, for most historians there's a limit to how immersed they get into other disciplines theories and approaches. This reflects a few factors, not least that it's hard to gain full proficiency in research methods from an entirely new field - it's not like historians have the time to do new degrees every time a new idea interests them, so we make do with scraping together some basic knowledge and applying it to our work as needed.

But more broadly, it also reflects a divergence in purpose that I think gets at what you're asking about. Virtually every other discipline sees the past as something to be instrumentalised, a source of data to address the questions you've decided are important. For social scientists, it's often explicitly because you care much more about the future - how can I construct a model using past data that will give me insight into what will happen next, and therefore allow me to make better predictions, shape policy or whatever else it is I want to do. Historians, on the other hand, are most interested in dealing with the past on its own terms - how do we use the past to explain the past? This means that we are interested in complicating rather than simplifying or streamlining what we know about the past, because the complexities are what draw us in in the first place. It also means that many of the specific theoretical or methodological issues other disciplines deal with simply aren't that relevant to us, so we can rather gratefully just skim over those bits and concentrate on stealing the stuff that seems useful.

This means that underlying purpose - and the questions that purpose leads us to ask - is probably the best way to define where the boundary of history ends and other disciplines begins. There's still overlap, most obviously to my mind with anthropology and literary studies, and a few holdouts in the other direction like economic history (a highly specialised subfield that has increasingly little to do with mainstream history writing). I've even seen it suggested that fields like International Relations are colonising what used to be the territory of currently less popular fields like diplomatic history (I personally think this is exaggerated - it's more that today's diplomatic historians are interested in different questions than IR scholars, so the latter need to do more of their own work). But while we definitely can't draw firm, permanent lines around history as a discipline, for most of us it's generally pretty clear that we belong as historians rather than something else.

It's a very vague line. Most social sciences borrow from each other in terms of methodologies. The fact that postcolonial perspectives is used in all is just an example of that overlap.

The most clear cut way to differentiate the fields would be with what kind of evidence is used. History concerns itself primarily with written texts. So in the most formal ways, as soon as you are analyzing a past text for evidence to construct an argument you are doing history.

In an academic setting the study of (European) antiquity is usually relegated to a Classics department because they use more approaches like archeology or linguistics that are not specific to history.

New forms of historical knowledge are also acceptable, like audio, video or photographs. Oral history is also really important as interviewing people can yield similar kinds of historical incites as texts. Interviews though are pretty commonly used in nearly all social sciences.

The kinds of questions aren't necessarily the same either. Anthropology is generally concerned with more granular incites into human behaviour and how different groups adapt. Sociology is generally more concerned with how more complex social behaviours develop. Both are a little more concerned with analysing the facts to extrapolate incites about human behaviour. History tends to stick to constructing evidence based arguments about how and why things happened. Few historians really express strong opinions about the present or future in their work, but again that's not necessarily always true.