The more extreme the reaction, whether it is positive or negative, the more likely it is that it is the illness.  If there are times when the person seems more rational, reasonable, and in control, and their behavior seems appropriate for the feelings they express, you could make sure you remember those times for later on, when the person may be more extreme.  The more you can learn about BPD, the better you will be able to measure the extremes as compared to more even-tempered times.  You should study the information available here and anywhere else you can find which will have good information.  Make sure the info you learn, though, is fairly recent, as attitudes and data about BPD have changed in recent years, compared to what it was a couple decades ago.  I have had BPD since I was quite young, over 45 years ago.  I have been "diagnosed" with many, many different problems over the years, but only lately have the diagnoses been of much use.  I find now that recent information about BPD is very helpful for me, while the stuff written even ten or fifteen years ago can be very wrong about a lot of things.  So be sure you study recent information!

Larry