I just used to blow hard on the end of the vent tube.And you would blow in the tube with spiders in there...ewwwwe
You can hear it whistling into the gas cap if it's OK.
The most common cause of blocking for me are spiders.
They crawl up the tube and make a nest.
I fixed that by fitting a piece of gauze over the end of the tube.
And you would blow in the tube with spiders in there...ewwwwe