https://gcc.godbolt.org is also helpful, it colorizes the assembly output corresponding to the C/C++ source. Furthermore, you can simply change between Intel and AT&T synatx.

The -S flag also works in clang, although the output may differ. For example, last time I checked a comparison check of i < 10 in a for loop would become:

  cmpl    $9, -4(%rbp)
  jle .L3

under GCC ('if i <= 9, loop') and:

  cmpl    $10, -8(%rbp)
  jge .LBB0_4

under clang ('if i >= 10, end the loop'). Neither GCC or clang does an exact conversion, and they both produce different assembly instructions (optimisations disabled in both cases).

If you build a cross-compiler, you can also output assembly for architectures other than your local machine, though this can be quite fiddly (see crosstool-ng for a project which has done most of the work for you).

GDB?