Fehling's test arrow pushing mechanism
The exact nature of the mechanism is unknown 

Be aware that some organometallic chemists consider arrow-pushing mechanisms inadequate/inappropriate for a variety of reasons (including the depiction of single electron transfer). For example, the arrow-pushing mechanisms we draw with alkyllithiums are incomplete in that they do not account for the oligomeric nature of the reagent.[2] 

The elements which are known:


The aldehyde or &alpha;-hydroxyketone must be enolizable. The enolate tautomerization of reducing sugars is discussed in McMurray[3]. This is common to the Benedict’s test mechanism as well.
The rate of enolization determines the rate of oxidation J. Am. Chem. Soc. 1970, 92, 537
Cu(II) has long been known to mediate oxidative coupling of enolates by a radical mechanism J. Am. Chem. Soc. 1971, 93, 4605. Mechanistic studies point to a metal-chelated single-electron-transfer process.5
The active species is the metastable [Cu(tartrate)2]6- ( Eur. J. Inorg. Chem. 2016, 1798). 


Drawing one plausible mechanism


Convince yourself that the Cu(II) complex  is a d9 17-electon complex.  That suggests you could plausibly draw an inner-sphere or outer-sphere mechanism. 
For the inner-sphere mechanism: coordinate the aldehyde, and generate the enolate (considering C-vs-O coordination and necessity of ligand-slip). 
Form the hydrate of the C-bound enolate, then homolyze the bond to form an organic radical and Cu(I). 
A second molecule of Cu(II) oxidizes the radical to the cation, which gets quenched the elimination of H from the hydrate to generate the enol form of the carboxylic acid.


For the basics of an outer-sphere single electron transfer see Mark Vander Wal’s Basics of Electron Transfer

[2]: Advanced Organic Chemistry: Part A: Structure and Mechanisms, Carey &amp; Sundberg, p. 413.
[3]: Organic Chemistry: With Biological Applications,  McMurry, p. 758