If I could give you a +1 Rep on starting a thread that is actually about seeking advice and improving your game, I would... However, I've neg repped you too many times
I agree with the previous two posters. The only way to know if they are interested or not is to talk to them.
Start slow.
Make eye contact with one girl, then do nothing else. Pat yourself on the back for that.
After you've made eye contact a few times with different girls, make eye contact and hold it. Cool, you're getting more comfortable.
After a few more girls, make eye contact and smile. If she smiles back, it's a bonus. But keep walking.
After a few more girls, smile and say hi. If she says hi back, it's a bonus. But keep walking. You're still just trying to get repetitions here.
Finally, once you're used to all of the above: make eye contact, smile, say hi and if they seem receptive stop and have a casual convo.
Don't ask them out.
It'lll just stress you out if you're out of practice - wondering when to ask for the number, if she has a boyfriend, is she going to reject you, etc. Just have a nice, brief chat and wish her a good day.
Do that a bunch of times. Pay attention for indicators of interest. Don't act on them. Just get used to noticing if a girl is interested.
Once you are in the habit of talking to girls, go ahead and ask for the number. From step 1 to here probably takes several weeks. Give yourself two months to be certain you've built up enough repetitions and gained enough confidence.
Good luck - and remember, if you hit bumps along the way, it's not the end of the world. The universe isn't against you. You don't suck. Keep building up your confidence and talking to women and you'll be ahead of 99% of the other "skinny" guys out there.
I don't think you'll get discouraged, though... if you follow the prescription above I think it will ease you into it enough that you won't overreact to rejections or interactions that don't go optimally.