READ OUT LOUD. Choose which marked text you prefer to practice with - macrons in verse form, or macrons in prose order, or accent marks in prose order, or focusing on the meter. You will find materials for all of these options below. :-)
VERSE MACRONS. Here is the verse text with macrons (note short final o in compatiendo, as typical of medieval poetry):
In campō sōlus dum forte iacēret asellus,

Et leviter digitīs pertractāns membra iacentis,

Hostī fallācī dixisse refertur asellus:

Vir sīc infīdus, fit quamlibet officiōsus,

PROSE MACRONS. Here is the same text with macrons written out in prose word order:
Forte, dum asellus sōlus in campō iacēret, lupus huic astitit, velutī compatiendō, et iacentis membra leviter digitīs pertractāns, inquīrit quae corporis pars mage doleat. Asellus refertur hostī fallācī dixisse: Pars quam tū tangis illa plūs mihi dolet. Vir sīc infīdus, quamlibet officiōsus fit, immo cum bonum facit, crēditur esse malum.
STRESS (ACCENT) MARKS. Here is the prose text with accents, plus some color-coding for the words of three or more syllables (blue: penultimate stress; red: antepenultimate stress):
Forte, dum aséllus solus in campo iacéret, lupus huic ástitit, véluti compatiéndo, et iacéntis membra léviter dígitis pertráctans, inquírit quae córporis pars mage dóleat. Aséllus refértur hosti falláci dixísse: Pars quam tu tangis illa plus mihi dolet. Vir sic infídus, quámlibet officiósus fit, immo cum bonum facit, créditur esse malum.
ELEGIAC COUPLET METER. Below I have used an interpunct dot · to indicate the metrical elements in each line, and a double line || to indicate the hemistichs of the pentameter line.
In cam·pō sō·lus dum· forte ia·cēret a·sellus,

Et levi·ter dig·itīs per·tractāns· membra ia·centis,

Hostī· fallā·cī dix·isse re·fertur a·sellus:

Vir sīc· infī·dus, fit· quamlibet· offici·ōsus,

IMAGE. Here's an illustration for the fable (image source) from a Renaissance edition of Aesop:

What follows is an unmarked version of the prose rendering to faciliate word searches:
Forte, dum asellus solus in campo iaceret, lupus huic astitit, veluti compatiendo, et iacentis membra leviter digitis pertractans, inquirit quae corporis pars mage doleat. Asellus refertur hosti fallaci dixisse: Pars quam tu tangis illa plus mihi dolet. Vir sic infidus, quamlibet officiosus fit, immo cum bonum facit, creditur esse malum.