Ever since the MOE stopped the ป.๖ test and instead implemented a "Thai equivalency exam" at the end of the year, there's been NOTHING which lets a foreigner know how proficient we really are in Thai.
The reason I say that is; with this new (5+ year old testing format) you're ranked against every one else who take the class
. This can and does lead to skewed results. If you sit the test with a bunch of dim-wit language learners 
you might score unusually high, where as if you sit the test with some ninja/samurai language learners 
you might score unusually low. It's still the same 4 tests, listening, reading, speaking & writing, it just doesn't provide the same "bang-4-the-baht" in terms of what you really know in my eyez. 
To me, when they stopped the 6th grade test it lost any real meaning; although there are more than a few schools which hawk courses on how to pass the new exam. Actually, I looked at some last week in a few schools and they're the same old courses but with different names is all.
It is my experience, teaching someone how to pass an exam is NOT the same as someone demonstrating proficiency in that subject, you just know the questions they're gonna ask is all. 
Be that as it may; The books I'm talking about are called แบบฝึกหลักภาษาไทย, they cover ป.๑ to ป.๖ and have 2 books for each grade. I bought some a few years ago but just recently picked up the entire set. They are set up with a synopsis of what the chapter is about covering the material which you're going to be tested on. There are exercises where you fill in tables and a final multiple choice for each chapter. Each grade covers mostly the same material with more depth of information provided and additional chapters of progressively harder stuff as you work thru the grades.
Now, they aren't gonna test how you speak thai, and if you can't read thai, you're gonna be outta luck too
but man, your reading comprehension, your understanding of grammar terms, or grammar in usage as far as how thai goes together will be put to the test! 
It also isn't put together the way foreigners are "spoon-fed" thai in language schools. This is formatted the way thaiz learn thai and it covers a LOT more stuff than we're taught! The hard copy boox are cheap, between 75-80 baht each so well within the budget of most self-studierz of thai.
The website which offers them is http://www.focus2552elearning.com
They are offered as ebooks (PDF filez) too, so you don't have to buy the hard copies if you don't want. Personally I'm a real book guy
, like to drag 'em around, work on them where ever I happen to be, but you could certainly print the PDF's and do that too. This is what they look like;
Here's what the first book of ป.๑ covers as far as material;
Just wanted to mention them, in case someone else was interested in what native thai language learners are expected to know grade by grade.
Good Luck,
Tod Daniels