SouthBend has the right idea. What I like to use is a 12V transformer, because this divides approximently 10:1 from 120V
line
Around here, I have several old HVAC transformers, but they are 240V primary. This is great, because I can use them on 120V, and have 12V output
(What you have should be simple, wait till you start screwing with 5KV plate transformers)
You need to isolate all windings with an ohmeter, there IS NO OTHER WAY.
Some transformers have tricky tapping and so on, so you MUST at least figure out which windings are interconnected.
On "fast" chargers and other high current transformers, both windings are such low resistance, that it would be impossible to tell what is the low and high taps.
Generally, the primary will be smaller, and there could be two of either primary or secondary, or two of each. Sometimes, for example, there might be two primaries, which are tied together to make one center tapped winding.
(Remember--that a "center tap" configuration usually uses a "full wave center tapped" rectifier, which gives you approx 1/2 the entire tranformer voltage, in the DC output.)
On some of them, you can carefully look at the way the winding taps enter the paper, with a combination of looking and using your meter.
Then, with a combo of first putting your 6-12V transformer into the primary, and measuring the secondary, and playing with taps (if any) you should be able to figure out what are the high and low taps.
If you want to be more sure of what you have--and be VERY careful--then hook your transformer to the secondary, and see what you have on the various primary taps. You will now see approx line voltage at the primary, so BE CAREFUL
When you first hook it up, put a 100-200 W incandescant lamp in series with the primary to limit current.
When you finally hook it up "for real" hook it up with a separate low amperage fuse, say, 5A-10A, with no load on the secondary. Listen for excessive buzzing and quick heating, which would indicate either
wrongly connected,
shorted turns in one winding
short to transformer frame.
Speaking of which, when you do all this MAKE ABSOLUTELY SURE that you have the frame of the transformer hooked to a good ground. There normally should be NO connection electrically to the frame (core) but if a winding shorts, it could short to the core, making it "live."