With nationalist issues, a good starting point is "does this fact / tale cast the cause the narrator supports in an entirely positive light or otherwise support the desired conclusion"? An argument that land historically belongs to the group arguing such is a good example of an automatically suspect claim. India is not unique in this regard: every state has its own mythos that reflects well on its claims and deeds. As soon as I hear a positive, uplifting tale about my own home, I think "yeah, we'll see about that". Don't get me wrong: this shouldn't be used as an opportunity for cheap cynicism, as historical actors can accomplish good things and sometimes things do in fact go well and without reflecting badly on those involved. But if you're wondering how to start recognizing a false claim, they often start here: uplifting your side.

The reverse of this is demonization. If you hear that the country / people your own group/state is continually opposed to committed a horrific crime, especially if with no apparent context or benefit other than just to be evil, that's worth double-checking too. And again, it might be true: history is replete with incidents of horrible cruelty. But this should send up warning flags, because justifying our own deeds by dehumanizing our enemy is a common political tool that trickles down into history, especially popular history and government-backed history.

I find that these two are the most common and most blatant historical sins seen at the popular culture / non-historian level. Beyond that it starts getting more complicated, and requires more knowledge / time invested in the cause of discovering the truth. You might have twenty or two hundred or two thousand works on a subject, and the only way to get a real satisfying feel for the field they cover is to start cracking them open and reading.

Broadly, I'd suggest starting with the most recent books, and working backwards. The older the books, the less reliable they tend to be, as a broad truism. We learn going forward, and also work to erase old prejudices that sometimes colour or completely ruin older works. (At the same time, we introduce new prejudices, but that can't be helped, and overall I think modern scholarship is stronger than older material). Check the bibliographies: what works keep being mentioned? Those tend to be the core of a field.

Look for books from university presses, with footnotes/endnotes, rather than popular history books, which can still at times be useful but as a rule are written by untrained enthusiasts of one stripe or another, or are simplified by professionals to crack a wider market.

Once you really dig into a field, sometimes you'll find a broadly-accepted single answer, even if this isn't an answer accepted in the same way by the public at large. Other times you'll discover that there's no consensus (though their might be majority and minority opinions). Eventually you'll learn what the trends are in a field, who is considered entirely respectable, who is considered broadly respectable but biased in X or Y key area, etc.

Ultimately this all comes down to research, research, and more research. To understand a seemingly simple thing properly, you often need a much larger body of information to draw on to give it context, to not just be able to say that X happened or didn't happen, but why, what led to it, and why not this other thing instead? This is why history is a field and we get paid the huge amounts that we do.

Best of luck.