How to determine max CPU speed of a processor in Linux?

If /sys/devices/.../cpuinfo_max_freq doesn't exist, that means the cpufreq driver isn't loaded - thus the CPU should be running at full speed. So yes, /proc/cpuinfo should be correct in that situation.

in addition to Marcin's comment: the second number in cpufreq-info -c 0 -l divided by 1000 gives you the max MHz