Sometimes stressing the tree out (by topping it or girdling it) is a good way to force it to fruit, and then you can see if the fruit is worthwhile, and if not, you can stump it and graft new shoots. Avocados sometimes suffer badly from girdling, so topping the tree might be something to try first -- I've gotten a seedling avocado that was ~10 years old to fruit by topping it. (The fruit wasn't any good.)