Summarize:

When you are at her place, wait for your girlfriend to become distracted by something that will keep her busy for a few minutes (e.g. talking on the phone, paying bills, walking the dog). Then sneak into her bedroom and peek in her underwear drawer. Or, if you want to win some bonus points with her, offer to do her laundry once it has piled up. Either way, gain access to her underwear and nightwear to find out what she needs size- and style-wise. Once you have access to her underwear, note down the sizes of her bras, panties, and any other lingerie she might own. With bras, be sure to note down both the number (chest-size) and the letter (cup-size). Also, make a note of the company that made each article, since sizing may vary between brands. If possible, find at least three bras and three panties that are each made by different companies. When you go shopping, provide this info to the staff so they can guide you with sizing if you want to buy lingerie made by a different company altogether. Now that you have her size written down, go through her undies again and get a sense of what she prefers. Write down which colors and fabrics she gravitates toward. Find out if she ever buys herself thongs or if she is clearly more comfortable wearing briefs and/or boyshorts. With bras, look for which style she tends to buy most often: full cups, half-cups, sheer, strapless, camisole-style, etc. While you are in her room, also check out the shades of makeup that she likes to wear as a helpful indicator of what colors she might enjoy in lingerie. Once you have a more solid idea of what types of lingerie she likes to wear, go through her drawer again. Keep a rough tally of how many articles of each type you find. Note the condition they are in, as well. Find out what she is running low on or will need to replace soon. Keep in mind that, while your girlfriend probably never wears a pair of panties more than once in between washes, she might wear each bra a few times before washing it. So if she has an even number of bras and panties, take that to mean that she may need a few more panties. If you want to buy her something beyond mere bras and panties, but she owns nothing else to point you in the right direction, propose a little dirty talk via phone, text, or email when you aren't together. Tell each other stories. Note what kinds of lingerie she chooses when she describes how she is dressed. Keep a pen handy in case she mentions anything you don't recognize by name.
Go through her underwear. Check her size. Gauge her tastes. Make a note of what she needs. Share fantasies to learn her tastes.