This should definitely be do-able, but there's not a specific API for it.
There are really 3 components to tracking this information:
Have they EVER connected - you'll know this already because if they connected they will have gone through the OAuth process on your website and you'll have a token for them.
Are they connected NOW - you can test this by making an OAuth request to any of Intuit's services - if you get back a successful response, you know they are currently connected, vs. if you get back a 401 error you know the token is expired/no longer good.
Disconnecting them - you can choose to explicitly disconnect a user (and thus no longer get charged for their usage) by using the disconnect API call.