Make sure they can talk confidently about:
- Difference between gross and net pension payments.
- Personal Allowance clawback.
- Understanding how SEIS and EIS operates for tax relief.
- Share schemes (this is a big one). At the least they should be asking:
- Share options or actual shares?
- If options tax approved or unapproved scheme?
- If shares any restrictions?
- If restrictions have you waived treatment on the market value at issue?
- Difference between when you pay Capital Gains Tax on the increase in share value and when you pay Income Tax (it's unlikely you'd pay both on the same amount).
- When you will also pay National Insurance on receipt of shares.
If they're thorough they'll hopefully ask about other sources of income, e.g. interest, rental, investment portfolios.
The top 2 should be bread and butter, 3rd should be pretty straightforward to someone who deals with Personal Tax routinely, the 4th is actually something I'd expect to trip people up as there is some complexity and specialist knowledge in there.