Why do you want to test? Leave it alone, let it do what it does best: ferment!
Are you thinking of keeping it sweet and not let it ferment out?
If the wine level in the fermentor is high enough to take a direct hydrometer reading, you could tie a string (use unflavored dental floss) around the top of the hydrometer. Sanitize it all well, then lower it carefully into your fermentor, take a reading and remove just as carefully. For all security you could slip a properly sized o-ring over the top of the hydrometer stem, to prevent the string from slipping off. Or even better, tie the string to the o-ring instead, so you can easily remove it.
You may need to recalibrate (and correct 1 point or so) for the weight of the o-ring, depending on its size.
Just be careful when working with hydrometers, they're very, very fragile. If it bottoms out in your fermentor, it will shatter, ruining your wine (glass shards).
Good luck!