You should know when it is appropriate to ask and when not:

Don'ts:

• complete strangers
• people who cannot retreat, e.g. cashiers, waiters and the like, on a busy train/bus in an elevator etc.
• people clearly not in a space to socialize.
• asking for sex
• being ambigious about intentions

Do's:

• people you held a normal conversation with before
• in a space where they are comfortable and either party can leave easily if things get awkward
• being clear about it being a date
• public place with individual privacy, e.g. going out for a coffee
• no alcohol or other drugs