• The primary coil of the transformer will draw the same power from the mains supply as the load is drawing on the secondary plus a bit extra for losses.
• Since P = V * I we can see that the high voltage coil will have low current and the low voltage coil will have high current.
• Therefore, the low voltage coil will have thicker wires and fewer turns than the high voltage coil.

Be careful. Use fuses. Wear goggles.

• The primary coil of the transformer will draw the same power from the mains supply as the load is drawing on the secondary plus a bit extra for losses.
• Since P = V * I we can see that the high voltage coil will have low current and the low voltage coil will have high current.
• Therefore, the low voltage coil will have thicker wires and fewer turns than the high voltage coil.

Be careful. Use fuses. Wear goggles.

[Update after photo added.] It's not possible to tell. There are no manufacturer's markings or labels on the wires. Options:

1. Go back to the UPS and figure out where the transformer was connected and figure it out from there.
2. Identify the coils on the transformer. Label each wire. Pick one wire and do a continuity test to every other wire. Write down what's connected to what - they're your separate windings. If you more than two wires in a group then measure the resistance between them and figure out which are the ends and which are the tap-off points. Probably both yellows are one coil, both blues another but why there's a bundle with blue, yellow, brown and red I do not know. There may be some centre-tap coils. Report your findings (in your question) in format "red - 7Ω - red", "yellow - 3Ω - brown - 3Ω - red", etc. or make a sketch and post a photo. e.g.,

^{simulate this circuit – Schematic created using CircuitLab}