A gift sweater, hat, or pair of socks loses its surprise if you ask the recipient to stand still for a tape measure. Sizing without direct measurements means working from indirect signals instead, and knowing which signals are reliable versus which ones just feel reliable.
Borrow a well-fitting garment
The most reliable indirect method: measure an existing garment the person already wears and likes the fit of, rather than guessing at body measurements directly. A sweater, laid flat, gives chest width (double the flat measurement), body length, and sleeve length directly, all without the recipient knowing anything was measured. This sidesteps the ease question almost entirely, since the borrowed garment's fit already reflects the ease the person prefers.
Standard sizing as a fallback
Without a borrowable garment, standard size charts (based on typical chest, head, or foot circumference for a given size label like S/M/L or a shoe size) are a reasonable fallback, especially for accessories like hats and socks where a size range fits comfortably rather than needing an exact match. A hat sized for an average adult head circumference (roughly 22-23in) fits a wide majority of adults reasonably well, since knit and crochet fabric has natural stretch most standard sizing already accounts for.
Building in forgiveness
When the size is genuinely uncertain, favor patterns and garment types with more built-in tolerance. A hat, cowl, or scarf carries far less sizing risk than a fitted cardigan, since none of those depend on hitting an exact circumference the way a button-front garment does. For a sweater specifically, choosing a size at the larger end of your estimate, or a style with more positive ease, gives more room for the guess to be slightly off without the finished piece feeling unwearable.
Asking without asking
A recipient's family member or close friend often has reliable size information (a shirt size, a favorite sweater's brand and size) without the surprise being at risk from the recipient's side. This indirect-but-accurate source is frequently more reliable than eyeballing someone's build from memory, which tends to under- or overestimate more than people expect.