The thing you need to know, is that the device should charge ok regardless of the amps (within reason), so long as the voltage is correct.
The caveat is that if you use to high an ampage, then you could reduce the battery life in the long run, to low ampage, and it will either take forever to charge up, or even could burn out the power supply.
If you look at the original charger for the device it will usually state the voltage, and ampage.
Another rule of thumb, is the higher the amp hour rating of the battery, the higher ampage that will be safe to use to charge it. E.G. A car battery would be around 60 amp hour, and a typical car battery charger might be 4 amp (upto maybe 10 amp for a rapid charger), phone's battery is likely to be 1.2 amp hour, (or 1200mah), and the charger might be 0.4 amp (or 400ma), so would take 3 hours to fully charge the phone from empty (actually longer due to increased resistance as the battery gets more charge in it, but it all gets a bit complex if you go into that).
A typical computer usb port is 5 volt 0.5 amp btw, so is close enough to most phones requirements to work.
Sorry that was supposed to be a quick simple bit of advise, but went on longer, and is probably less clear than I intended.