What are your temps? If you watch the airlock you will see the ferment build for a few days, hold steady and then start to slow down. When it starts to slow down is usually when to check. Sanitize the thief first, it only takes a few seconds to read the sg
With a thief, you take off the airlock with one hand and check the gravity with the other hand. As long as you dont let the airlock touch anything, you just put it back on and your done. Keep a cup next to the carboy so you can drop your theif in it, along some of the cider sample for tasting.