How can I see what kind of car charger I have?

I noticed that when using navigation plus GPS my phone (Galaxy S2) still discharges a bit even so its connected to a car charger. While googeling, I found out that regular USB chargers only charge with .1 amp but since a car battery isn't drained so easily, there are also car chargers that charge with .5 amp. Is there a way see on the phone how much its beinbg charged with ? My phone only displays "USB" or "AC". I'd like to find out if buying a "fast charger" would make any difference or if I already have a charger that supplies .5 amp.