How do you recognize a loop in assembly?

Question

Accepted Answer

Look for a backward conditional jump: a label, a block of body instructions, then a cmp/test + conditional jump that targets that earlier label.

* The tell-tale sign is a backward conditional jump. do-while jumps back from the bottom; while adds a guard jump that skips the body; for adds an init before the loop. *
.L_loop:
    # body instructions
    addl $1, %ecx
    cmpl %edi, %ecx
    jl   .L_loop

Types of loops by structure:

Loop type
Pattern

do-while
Body first, conditional jump back

while
Initial conditional jump to skip, then body + jump back

for
Init before loop, body + update + test + jump back

while loop has a guard jump before the loop:
    cmp ...
    je  .L_done
.L_loop:
    # body
    cmp ...
    jne .L_loop
.L_done:

Identifying the loop variable: Look for a register that gets incremented (add $1) or decremented (sub $1) near the conditional jump. The cmp before the jump is the loop condition.
Go deeper:

Control flow — loops (Wikipedia) — Defines while, do-while, and for loops, matching the three structural patterns in the answer.
 Jcc conditional jumps (felixcloutier) — Reference for the conditional jump that forms the loop back-edge.

Loop type	Pattern
do-while	Body first, conditional jump back
while	Initial conditional jump to skip, then body + jump back
for	Init before loop, body + update + test + jump back