I can answer at least for the northeastern end of India.
Today the control of the borders is largely done by special military organisations. The Assam Rifles are the group that handles Arunachal Pradesh and Nagaland, and who in addition to the APP manage checkpoints into Arunachal. But for the people from the region, there is no border. I'm speaking pre-1997 here, but it applies today too. India has had an absurd amount of conflict going back decades which I wrote about here last year for an overview. Much of this stuff is ongoing today.
As part of my work I end up digging through a lot of British surveys of the border area, many of which can be found in the street markets of Yangon (a.k.a. Rangoon. Go to Pansodan Road's bookstores if you're ever there). These are overviews of tribal groups and mountain paths in the then-called Naga Hills or Frontier Tracts, the areas of the Patkai and lower Himalayas where what's now Northeast India met British Burmah and then also China in the north. I also work closely with old British survey maps of the area going back to the end of the 1800s, and spend half the year in these jungles in the places on these maps.
Simply put, these borders weren't guarded at all. They weren't surveyed until much later, and the maps we have from the 20s and 30s were actually pretty inaccurate. But that's understandable because this area is dense jungle, and if a village gets abandoned, it's invisible in 10 years thanks to jungle regrowth. Groups living in the areas were at times friendly to the British and at times hostile. This is mentioned in the surveys as well, with statements like "this group has been helpful in the past but wars with group X nearby and may not be friendly today". Names of village/clan heads are listed, as are how many guns they have.
For the most part, the British simply didn't go up that way. Prior to WWII, the so-called "Hill Tribes" were left largely to their own devices. Even today, neither India nor Myanmar have much idea what goes on in the border region, and since it takes days of travel on foot on muddy jungle paths to get to a lot of these places, that's unlikely to change in the near future.
Most of the groups I work with on the India side of the border migrated from the Myanmar side in the past 100 years, many under British rule. Others migrated in the 1960s and continue to move back and forth over the borders freely to today. Post-Indian and Burmese independence, the porousness of the border was (and continues to be) a major issue for smuggling of things like guns, opium, and people.
I mention things as they are today, because at present there's little actual control of the borders, and that matters because it's actually only gotten more controlled over the past decades, so the minimal control that exists now is a good indicator of how it once was. Mods forgive me for pushing the 20 year rules.
Sources:
Unfortunately as I mentioned the studies I'm basing this off aren't available online, but if you just so happen to live in the UK you might be able to find them at the library, or, you know, in used book stores in Yangon.
tl;dr: The border between India and Burma and between both and China were basically un-guardable. The terrain was/is rough, the infrastructure was nonexistent, and the people who live in the area were always better at moving back and forth than the State was at preventing that.