It's pretty easy to reach a level in Esperanto where you are pretty much guaranteed to reach 100% in any test of grammar. So such a test is not very exacting.

But there is more to mastering Esperanto than just knowing the grammar.

One high-level test would be of whether you have an actively rich vocabulary.

For example, how many near-synonyms of 'grand(eg)a' can you think of? If you can think of more than 20 you doing well.

Hint: think of this idea as applied to different objects.

sudanglo:
For example, how many near-synonyms of 'grand(eg)a' can you think of? If you can think of more than 20 you doing well.

Hint: think of this idea as applied to different objects.

The thing is in Esperanto this would be more a test for your creative thinking than your vocabulary. Indeed, you can create a lot of synonyms whithout knowing a ridiculous amount of roots. In his book "le défi des langues", Piron mentions an excercice in a classroom where the pupils were asked to think of as many synonyms for "li estas stulta" as possible. Despite having studied Esperanto for only one year, the kids came up with a total of over 100 phrases.

In your proposed test, a student with limited vocabulary but enough creative thinking could probably hit the 20-word mark just adding -eca to roots refering to huge things like "elefanteca", "nuboskrapuleca", "kontinenteca", etc. If -eca is not regarded as precise enough, then they would use -dimensia, -skala, or -ampleksa instead.

Another, less metaphorical tactic would be to combine alt- with any kind of measurement words: alt-skala, alt-mezura, alt-grada, alt-dimensia, alt-kvanta, alt-kioma, alt-ampleksa etc. For these you would already need a bit more vocabulary, but not so much more either.

20 synonyms and almost-synonyms (without looking at a thesaurus):

ampleksa
behemota
cxielskrapa
cxioma
ega
epopea
giganta
grandioza
kolosa
kosmoskala
largx(eg)a
makroskopa
maleta
malliliputa
malmikroskopa
masiva
profunda
tre(eg)a
vasta
volumena

You make a valid point yyaann. So give more points in the test for words like enorma, giganta, ampleksa, vasta, profunda and so on.

I didn't think of elefanta. Good one!

Another way of testing it would be to give a list of nouns and ask the testees to suggest appropriate qualifiers with the meaning of grand(eg)a.

For example, prezo, scio, lumo, frosto, temperature, apetito, libro, monto, vojaĝo etc.