Will a caution show on a CRB check?

It depends how long ago it was issued and what the caution was for:

Cautions will always show up on a DBS check if they are for an offence that is included on the 'DBS list of offences that will never be filtered from a criminal record check' (available on the government website).

If the caution offence is not on that list, it shows up for 6 years after the time of issue (2 years if the individual was under 18 at the time of caution). Once that 6 years (or 2) is up, the caution becomes 'protected' under current legislation and no longer has to be declared to employers.

It's worth noting that for job that are not eligible for DBS checks, you only need to declare unspent convictions and do not need to mention cautions at all.