Refer to figure.
ICAO Annex 10 Volume II
5.2.1.5 TRANSMITTING TECHNIQUE

HOW DO YOU READ → “What is the readability of my transmission?” (see 5.2.1.8.4.)

5.2.1.8.4 PANS.— When the tests are made, the following readability scale should be used:

Readability Scale

1 Unreadable
2 Readable now and then
3 Readable but with difficulty
4 Readable
5 Perfectly readable

The phrase "how do you read?" is a standard phrase used by pilots and air traffic controllers to inquire about the quality and clarity of a radio transmission. When someone asks "how do you read?" they are essentially asking if their message is being heard clearly and if the recipient can understand it well in terms of loudness and lack of any background noise.