Yes, you can certainly estimate the query cost before you actually run it.
When you enter a query in the GCP Console, the query validator verifies the query syntax and provides an estimate of the number of bytes read. You can use this estimate to calculate query cost in the Pricing Calculator.
If your query processes a small amount of data, you might need to convert the bytes that are processed from KB to MB. MB is the smallest measure that is used by the Pricing Calculator.
For various other methods that you could use for estimating the query price, refer to https://cloud.google.com/bigquery/docs/estimate-costs#estimating_query_costs