If you put out technical documentation you're unsure of, then there's a big problem. Chances are if you're not certain of some content in a document, then your audience may be uncertain too.
Unless you're dealing with theories or scientific findings in white papers, technical documentation must be a rock of certainty to the user. Why? It's because you're guiding your audience to do something. If you're not instructing them, then you're guiding them along to explain some information. In either case, you shouldn't fly blind on what you're writing or editing before you publish.
So how can you make sure you understand what you're writing?
Understanding the big picture of a product or service of what you're about to document is extremely important. If you don't have an idea of what you're writing about, it will show.
If you sit down with an SME, have them tell you briefly what something is. Once you get a gist of what you're writing, the rest is just taking the next steps. Don't get hung up on details. Those will follow as you move along. It's more important to take a step back to see the bigger picture, so you know how to create a document.
Understand your audience
It's important to understand so you would know who are you are writing for. What kind of information will you include? What information not include? For example, if you're writing for a software company that helps electrical engineers create schematics of an electric circuit, you should find out what schematics and electrical circuits are before you proceed. However, you wouldn't explain what these things are to an electric engineer since they should know. You simply explain to them how to use the software.
For more about knowing your audience, please check out what I said previously.
If you're unclear on what you do, please ask questions. Ask and never assume. No one should expect a technical writer to be an SME. Do as much as research you can on your end. Then, ask the SME questions about the product or service you're uncertain about. Any SME worth their salt will be able to answer your questions. Don't be afraid to ask. It's better to ask plenty of questions and put out an accurate and clear document, than to put out a document that's unclear, inaccurate, and full of uncertainties.