Named employer or company
Most legitimate corporate enquiries come from a named company email (john@bigcontractor.co.uk) not a gmail account. The legitimate contractor enquiries that ARE on personal email usually mention the contracting employer in the notes.
Specific dates and headcount
Vague enquiries ('sometime in spring, maybe 4-6 people') are usually time-wasters. Specific enquiries ('check-in 3 Feb, 4 weeks, 4 men') are usually real.
Reasonable budget expectations
A request for £80/night for a 4-bed house is unrealistic. Polite decline. A request at or above your published rate is the green light.