I've been extracting DMT for over a decade.
A lot of people have never had real pure DMT. In fact, even on the dmt-nexus forum, most people there do not even seem to know the difference between pure DMT and DMT mixed with NMT and other tryptamines.
Part of the reason for this ignorance is that for many years, until a few years ago, pretty much whatever Mimosa hostilis/tenuiflora bark you bought would contain some fairly pure DMT (high 90s percentage-wise and felt basically like synthetic DMT). However, starting maybe 3 years ago, I've noticed it is very hard to find Mimosa hostilis root bark (MHRB) with that same pure alkaloid profile. I was finding a lot of interesting and novel tryptamines, and had some of them verified by LCMS. Sometimes, the bark had no DMT at all from what I could tell, but usually it was something like 20-60% DMT.
Now, how can you tell whether you have pure DMT if you don't have a lab to test it? Some people would say Thin Layer Chromatography, but let me tell you, that is not very quantitative and the results are pretty variable with it in my experience.
But luckily there are some pretty simple ways.

1. Smell it. DMT has a very distinctive floral smell which is really lovely. Some say it smells like roses. That's not exactly descriptive because different roses have very different smells and most roses these days that you buy from the market have almost no smell. (The smell of real wild roses is absolutely amazing.) Other tryptamines that I've found in Mimosa hostilis did not have that smell at all, and either smelled like nothing, or had a fairly neutral smell or sometimes an unpleasant smell. There was one tryptamine in Acacia confusa which I nicknamed "metta" because it was an amazing heart-chakra opener like MDMA but more psychedelic (if you search for Metta NMT discovery on google you'll find about it), and that did have a really lovely smell of jasmine, but it wasn't the same smell as DMT.
   
2. Vape it. Pure DMT has a heavy body load, is harsh on the lungs and throat compared to the other tryptamines I found, and most importantly, it quiets all thoughts. Thoughts gone. It blasts open your third eye and crown chakra and dissolves your ego with a fearful force. If instead of that you feel that you are ruminating or that there is a lovely vibe to it, that's likely NMT that you are feeling. NMT does not do much unless you pre-dose harmalas, so you might not notice NMT effects that much if not using harmalas. Btw, DMT is a waste of time without harmalas, though you'll certainly feel it. There's a reason that DMT is traditionally considered to be the "light" to see the "teaching" which the harmalas provide.
   
3. Crystalline structure. This one is not as reliable because tryptamines can form different crystalline structures, but pure DMT often has a kind of snowflake fluffy appearance, while other tryptamines that come out will often have a hard blob appearance or a needle-like cluster form.