You can vaporize small amount (avoid burning it!) and see how much residue there is. Pure crystal will vaporize perfectly and leaves only crystal behind. Impure DMT will leave oils behind.
If you test it by bioassaying, you can try threshold dose and see how active it is. Also the smoother the vapour, the purer the DMT.
You can dissolve your product in a minimum amount of warm naphtha or heptane - impure DMT leaves behind oils and solid impurities.
You can titrate known amount of DMT with acid, this will tell you % alkaloid content in the sample.
You can recrystallize the sample and compare weight before and after. The higher the yield, the purer the crystal.
Observe melting point range - this is a popular method of quick purity determination. But it requires an expensive apparatus.
And finally, you can send the sample to lab for GC-MS analysis. They will tell you how much % DMT you have in your sample.