In general, two tones that are close together in pitch will sound dissonant, while two tones that are further apart will sound consonant. This is because the ear perceives the difference between two tones as a "beating" or "pulsation", which can sound harsh and unpleasant if the tones are too close together. As the interval between two tones increases, the beating becomes slower and less noticeable, until eventually the tones sound consonant.
The exact interval at which two tones start to sound consonant varies depending on the individual listener and the musical context, but some common intervals that are considered consonant include the octave, the fifth, the fourth, and the major third. These intervals are all based on simple ratios of frequencies, which makes them sound pleasing to the ear.
It is also important to note that the perception of consonance and dissonance can be affected by the context in which the tones are played. For example, a tone that sounds dissonant on its own may sound consonant when played in a chord with other tones. This is because the other tones in the chord can help to mask the dissonance of the individual tone.
Ultimately, the best way to determine whether two tones sound consonant or dissonant is to listen to them and see what you think. If the tones sound harsh and unpleasant, they are probably dissonant. If they sound pleasing and harmonious, they are probably consonant.