A all-questions-welcome resource will, almost inevitably, be less helpful than finding pertinent resources for the specific topics you're asking about. And that'll be a case-by-case process.
The general workflow will be:
- Figure out which field your question falls into (e.g., physics; architecture; survival skills)
- Google "[Field Name]" together with keywords such as "community," "forum," etc.
- Browse through the results until you find something that seems appropriate - some site on the topic with a public forum; a newsgroup; a LiveJournal community; a chat room. Make sure to read through to be certain your question won't be wildly out of place.
- Post your question. Explain that you're looking for detail for a story you're writing; be sure your question is clear and detailed; be polite and appreciative. Mention that you'd be happy to hear of other appropriate resources.
This process is great for a single detail, but if you find that your work is dealing heavily with a particular topic, you really need to do your own research - you won't get by giving a fair portrayal of desert survival or Victorian England just by asking other people about the occasional detail. For greater involvement, you want to find good sources on the subject - generally books. You can search yourself for books that cover the topics you need; if you're not sure or want a second opinion, you can follow the above process to ask for specific recommendations. Be sure to explain exactly what topic you're researching, and what kind/style/level of books you think will be best for you.
Beyond this, there are many communities based on answering specific questions on a wide range of topics. These are certainly worth trying out, particularly as an initial avenue. These include: