There are a lot of signs you can look out for. The most obvious I suppose are from girls close-by (up to about twelve feet) hairplay, getting carried away in conversation, holding eye-contact, getting fidgety, checking you out - a lot, blushing like a beetroot or the absolute giveaway neck flushing!
However there are many. many more signs, depending on the individual woman. This thread is a good guide:
As for where to use Pheros - Ammo is a well balanced product, so can be used very widely in work/college/polite social environments, compared to the more overtly sexual products like TUTH, which are too overt and can be taken as creepy, aggressive or just too horny if worn out of context. Save these for clubs, parties or other situations where it is acceptable to be a bit more raunchy.
Similarly, the sort of environment you are in will affect the response. Work/class/sedate environments tend to produce more restrained responses compared to clubs and parties or any other situation where inhibitions get lowered. Saying that, one of my very first phero hits was when a colleague out just started talking about sex, quite out of the blue and later that day, another woman started to tell me all about her sex toy collection! 
You might also want to read these thread, which deals with all the basics of phero use:
Yes, Ammo is a very good place to start, along with any of the socials IS, IO, IH) and IG - you will find your way with these most easily. Also, do not underestimate the socials when it comes to younger women - They do tend to be attracted to a more broad range of social/sexy feelings compared to older women (late 20s seems to be the changing point towards a more overt sexual/status signature) and these products are very valuable for promoting all the right feelings across the board. 