Condensation between the glass is a big sign of failed double glazing, but there are other signs too.

Source: https://www.express.co.uk/life-style/property/1717518/condensation-windows-double-glazing-ifl

The elevated property benefits from gas central heating and double glazing throughout, and there is on-street parking available directly adjacent to the front door.

Source: https://www.clydebankpost.co.uk/news/23653144.clydebank-property-duntocher-villa-perfect-family/?ref=rss

The interest group would particularly like to see double glazing made mandatory, or single glazing become an official defect, so tenants can go to court.

Source: https://houstonianonline.com/private-landlords-rarely-use-subsidies-for-sustainability/

The property benefits from double glazing and a combi gas central heating system.

Source: https://www.southwalesargus.co.uk/news/23976740.still-time-grab-festive-bargain-property-auction/?ref=rss

The property benefits from oil-fired central heating and most of the windows have sealed unit double glazing.

Source: https://www.darlingtonandstocktontimes.co.uk/news/23361315.property-former-chapel-near-harrogate-thirsk/?ref=rss

"The property is equipped with gas central heating, double glazing, and top-quality floor coverings throughout.

Source: https://www.ayradvertiser.com/news/23962304.dalmilling-three-bedroom-semi-detached-villa-sale/?ref=rss