I have a Mac, so mine came with one. I had to download a separate one to use in my earthlink email. I'd expect to be able to find it in your tools, but if you can do a "find file" search, (Is that right? It's "Finder" in Mac.) for "spell check" or maybe "spellcheck" and see if anything comes up. If it doesn't, you can check the updates/software website for your model computer, you can google to find those, and do a site search for spell check. You can most likely download one for free.
It's been a long time since I've used a PC, so I can't tell you much about those. Perhaps a computer-savvy PC owner will speak up and have better info than I do. You can also google "spell check software" and if you wish, "grammar check software" , or "free spell check software" (or "freeware spell check", etc.). You'll most likely find quite a few. Mine give me the option of underlining errors in red as I type. This is very helpful to me, as I get in a hurry and leave out letters. I also need to clean my keyboard, the keys are a little sticky, sometimes they don't register when I tap them.
Without spell check, I would no doubt, post a lot more errors. Even with it, a few slip through. Posting late at night, while tired, increases errors, too. I'm sure lots of us are frequently posting-while-groggy.