I have an addition where the toilet clogs every few weeks. You can hear it coming a few flushes before the back up with a LOT of gurgling in the pipes. I "fix it" by running the shower, and air is forced up and out of the toilet, and then a few minutes later the toilet drains and is unclogged.
After snooping here, I suspect something is improperly vented. But where do I find the vents? Are there separate vents for toilet and shower?
Your vent, if it exists, should reach the roof (there are exceptions, as noted in comments). You can look at the roof for a short metal or plastic pipe sticking out, it may or may not have a cap. This would give you a rough idea where it is inside the house, it is likely inside a wall. Depending on the layout there may be more than one vent.
If it does not have a cap it could be plugged by something (birds, squirrels, intercontinental coconuts) falling into it - this would likely be apparent with more plumbing issues (gurgling, slow draining) at more than just your toilet, though - if there is more than one vent it could affect "half the sinks/toilets" or be limited to "just one bathroom's devices".
If you can find the vent and find a clean-out as suggested in a comment, you can try to clean it out. Have a bucket under the cleanout in case "stuff" comes out as you're working.
