Tax covers compliance and planning work: returns, filings, and structuring decisions made ahead of them. Providers range from firms handling straightforward annual filing to specialists in a particular regime, industry or transaction type.
Jurisdiction is the first filter and it is not negotiable — tax expertise does not transfer across borders, and a firm competent in one country's regime is not automatically competent in another's. After that, establish who signs and who is liable, what happens in an enquiry or audit, and whether the fee covers that or is billed separately when it happens. The answer to that last question is worth having in writing.