Note: This is written from the perspective of a postgraduate student in applied mathematics.
1) Do not cite Wikipedia.
This is not about perception or laziness but rather, your thought process as a researcher. Suppose I read about a mathematical fact that might be useful to my research. I need to verify that the fact is true and have some ideas about why this is true.
By stating that the mathematical fact has been published in a reputable peer-reviewed journal or in a reputable textbook, I demonstrate that I have at least verify its authenticity, and perhaps even read technical details about it.
However, if I cite Wikipedia, it demonstrates that I accept facts off the internet without verifying or having technical understanding about it (Wikipedia usually don't go into deep technical details). This does not bode well for my reputation as a researcher.
2) Try to find an academically acceptable source to cite the same information from.
Suppose I want to use an equation. Random example: Kullback-Leibler Divergence. But lets pretend there is no source or citations on Wikipedia.
What I will do is to search directly for "Kullback-Leibler Divergence" using search engines like Google, Google Scholar or Google Books. I will also try to search for the term in my university or local library's search tool.
Assuming this fails. Then, I would look at topics that the Kullback-Leibler Divergence is in or is related to. For this specific example, I would look for textbooks or materials on "Information Theory" and look up their index or table of contents for "Kullback-Leibler Divergence". If this fails, I will dig deeper: think about what this equation does and search for similar topics. For this example, it compares two probability distributions. I will then look for ways to compare two distributions in Information Theory.
Once I find a paper or textbook talking about it, it shouldn't be too difficult to locate the source or pick a suitable paper/textbook to cite the equation from. If after all these searching and perhaps asking my supervisor/professor, I cannot find anything acceptable to cite from, I would ask myself these questions: Is this equation valid? Why should I believe in the authenticity of this equation?