to spot.
You have almost certainly written one if your sentence contains more than 3 or 4 conjunctions. If you read the sentence aloud and run out of breath before reaching the end of it, you have written a rambling sentence. If your sentence stretches over many lines of writing, you have certainly written a rambling sentence and most probably a run-on sentence too.
Unlike run-ons or fragments, rambling sentences are not wrong, but they are tiresome for the reader and one of the signs of a poor writer. You should avoid them.