Check the front while seated
Fasten the waistcoat and sit down. The buttons should not pull uncomfortably or create gaps you do not want. If you intend to wear it without another top, check the armholes and neckline from several angles before deciding the coverage suits you.
A light shirt or T-shirt underneath changes both the coverage and the fit. Try that exact layer rather than assuming it will fit because the waistcoat closes on its own.
Separate it from the idea of a suit
Jeans can give a tailored waistcoat a more relaxed setting. A plain skirt or a different-coloured trouser can work too. If two fabrics nearly match but not quite, compare them in daylight and decide whether you prefer a more obvious contrast.
Look at the hem where it crosses the waistband. Pointed fronts can leave a gap at the sides, while shorter straight cuts may reveal more of a high waist. That is a detail to test, not a reason to choose one universal length.
Add the layer for the journey
A waistcoat may feel complete indoors but still need a jacket or coat outside. Put that outer layer on and check the shoulders and back. Extra tailoring beneath a close-fitting coat can become restrictive.
If the waistcoat has an adjustable back strap, use it gently. Tightening it cannot solve every fit issue and may simply pull the front out of shape. Keep enough room for ordinary movement and let the garment retain the outline it was made to have.
