I think the answers are the same as on the other question, but let me try and be coffee specific. I should also note that I have about that exact situation and I still go to Starbucks everyday and have never tried the other places, but I know I like Starbucks coffee (and I know not everyone does and there are probably better places). Assuming I had wanted to know what's best here's what I would have done -- in rough order of preference.
Ask someone, either the hotel / hostel staff, random people on the street, other travelers, etc. I've been asked several times by strangers -- particularly in tourist areas -- where the best place to go for this or that. Or just ask in the shops themselves about what their beans are, what they sell -- it should be easy to judge how much they care about what they're doing from that.
Observe where everyone else goes, what's busiest? Ideally pick a good coffee drinking time but be aware of external factors. Here Starbucks gets pretty busy around payday or when they're running an offer, the rest of the time it's much the same as the other places. Also observe the type of people drinking there, look for someone that looks like the might like the same things you do.
Check the prices, lurk around and look at the menus. Most coffee shops will let you browse for a bit so see if you can see what other people are paying (or just ask). Does it just do coffee and a few other things (which means they probably care about the coffee) or does it offer a wide range of stuff, with many non-coffee drinks and food. Are they selling more standard coffee, or lots of syrupy flavored coffee drinks? Do they sell coffee beans as well. Do they have a sign saying what coffee they sell?
Do you have internet access? Google the places, have a look at what the website says if they have one. Or look at on-line review sites, even if it's not in English should should be able to decode the rating system.
Finally, depending on how long you're there and how much coffee you drink, just try them all, ideally at least twice. It's the only way to be sure.
This wasn't in your question but there may be other considerations as well, depending if you're going to stay there a bit. Do they have wifi, comfortable seats, outside seating, a toilet ...