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.