We can’t know for sure what tunes were used for the Wesley hymns originally. Many people think that contemporary songs from the tavern or pubs were used, but that isn’t true.
To select a tune today for a hymn what you can do is take the poetic meter of the hymn text and find a tune that has the same number of syllables per line. Then you can sing the hymn to that tune.
If you look in The United Methodist Hymnal one of the indexes in the back of the book is for hymn tunes indicating the number of syllables per line. You can simply choose a hymn tune to sing. However, you should also look carefully at how the text matches the rise and fall of the melody. Consider the musical high and low pitches, if ‘unimportant’ words such as the articles “a, an, or the” fall on longer notes, notice how the whole melody ascends or descend, where odd or insignificant words might fall in a more significant portion of the melody (look at the length of time for the note or its pitch within the melodic phrase). You need to carefully evaluate the melody to assure that the tune you choose really fits the text. For example, you don’t want a text that talks about climbing the heights to glory and yet the tune has descending notes for that part of the hymn.
If you look in the back of The United Methodist Hymnal you will find several indexes. One is for metrical settings, another is hymn tune names. These become important if you’re trying to set a text to a new tune. If you know the hymn tune by name, such as “Hyferdol” you can look up what the metrical setting is to see if it matches the text you have in mind.
Or, if you ascertain the number of syllables per line of a poem, for example, 98.98 you can find a hymn tune with the same meter. The first line of a 98.98 song will have 9 syllables, followed by a line with 8 syllables. For example, UMH 624 “Bread of the World”.
Bread of the world in mercy broken, wine of the soul n mercy shed, (9 syllables)
by whom the words of life were spoken, and in whose death our sins are dead. (8 syllables)