Connect A to E and measure between B and F. You can do something similar with the third winding. You can connect the windings this way to double the voltage or swap C and D to have the windings in phase again. If the voltage measured between B and D was 38V the windings are in anti-phase: if A is the start of A-B then C is the end of C-D. If the voltages are equal you can place them in parallel to double the current. If this is 0V (very low anyway) A-B and C-D are in phase, so if A is the "start" of A-B, then C is the "start" of C-D. Now connect A to C and measure the voltage between B and D. In my example rated voltages might be 2 x 12V and 8V.
Those are unloaded voltages, and especially for toroidal transformers those may be much higher than the rated voltage. Measure the AC voltages of A-B, C-D and E-F. To find the polarity you'll have the connect the primary to the mains. Let's say you can find windings A-B, C-D and E-F this way. If you measure mega-ohms your measuring on two isolated windings.
Depending on the transformer's rating you'll measure a few hundred ohms between begin and end of a winding. To find start and end of a winding you measure resistance with your multimeter. For the secondaries I presume they're separated. If you get a negative voltage reading, you know you have the test leads swapped.The primary's wire is thinner and as W5VO says has a higher resistance. The only way to be sure is to use a voltmeter and measure the voltage across the two wires. *I say " usually" since I've seen a wall wart with the wires were reversed, although every other wall wart I've used does it the way I've described above. text providing wire information, a stripe, etc.) is the positive end, and the unmarked wire is the negative end. This kind of convention is used on speaker cables as well, where the wire that is marked in some manner (e.g. It doesn't matter if it is striped or dashed, the presence of any kind of marker is the indicator of the wire being the "positive" end of things, as opposed to the unmarked "negative" wire. Usually* the wire with the white stripe or the dashed lines carries the "positive" (+) end, while the other, unmarked wire carries the "negative" (-) end. The solid/dashed lines on wires like the ones pictured in your question are used to indicate polarity e.g.
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