Is there any way to distinguish between single and multi currency QBO user?
Check the details here- https://developer.intuit.com/docs/0025_quickbooksapi/0050_data_services/020_key_concepts/currency
You need to query Preferences endpoint-
<Preferences domain="QBO" sparse="false">
<Id>1</Id>
...
<TaxPrefs>
<UsingSalesTax>false</UsingSalesTax>
</TaxPrefs>
<CurrencyPrefs>
<MultiCurrencyEnabled>true</MultiCurrencyEnabled>
<HomeCurrency>AUD</HomeCurrency>
</CurrencyPrefs>
...
</Preferences>