u know, living together and being together is different. life before and after marriage is also different. hence, there is really no fool proof way of telling whether one's your lifetime partner.
well, in a way, understanding you during difficult times may not be totally sufficient. perhaps sharing the burden, sparing her shoulder for you, helping out if possible, and going that extra mile to make sure you're alright are some of the things she might do if she's the one for you.
time is also another factor. contrary to popular belief, i personally think that time is one major factor. nothing beats the time needed to get to know you, understand you, and feel you better. of course, there will be a time when you decide that your relationship has matured enough to settle down. that will be the time when you can pronounce that "time is not an issue", 'coz anything from then on will mean the same.
difficult times may be a blessing in disguise, as such situations will really test the maturity of your feelings for one another. least that's how i think.
you're still so young........ be patient....... love and settling down should not be your primary concern now. studies, national service, and career will have you breathing down your own throat!