Last step of nicotine extraction using DCM
If you are worried only with DCM, then you should be fine since it is quite volatile. To be sure you have no DCM left in your nicotine leave it for a few days in a closed recipient filled with some dried silica gel or calcium chloride (here in my country those are easily found, even for domestic use), and if you are in a proper laboratory just leave it in the desiccator with the vacuum on for a few hours.

However, nicotine is hard to isolate since other alkaloids from tobacco are also extracted in the most commonly used methods. I don't know which extraction methodology you used but just for comparison, here are two papers with some information for isolating nicotine:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3263647/
https://www.ncbi.nlm.nih.gov/pubmed/19135499