Compare the leaves and the site it's growing in: are the leaves noticeably small and fine, and is it growing happily in a fairly sunny spot? Or are the leaves larger and coarser, and is it tucked into shade or part shade?
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If you are still unsure
If you can't tell japonica from sasanqua and don't yet know the flowering month, site and treat it as if it were japonica — sheltered from early morning sun — since that's the safer default for both groups. You can relax the shelter once you've confirmed it actually flowers in autumn/winter rather than winter/spring.
COMMON MISTAKE
Assuming every camellia needs the same shade and frost-shelter as the classic spring-flowering japonica. Many sasanqua types tolerate full sun once established, so over-shading one just needlessly limits its flowering — a much smaller problem than the reverse mistake, which is planting a japonica in an open, sunny, east-facing spot: fast morning thaw after frost scorches its buds and open flowers, the classic cause of a japonica that 'never flowers properly'.
Already know the variety name?
The name on the label tells you the type.
Camellia japonica ‘Adolphe Audusson’Camellia japonica › · Rich red, semi-double, large flowers with prominent yellow stamens.
Camellia japonica ‘Nobilissima’Camellia japonica › · White, formal double, peony-form, one of the earliest japonicas to open.
Camellia japonica ‘R.L. Wheeler’Camellia japonica › · Rose-pink, very large semi-double/anemone-centred flowers up to 15cm across.
Camellia japonica ‘Bob’s Tinsie'Camellia japonica › · Bright red anemone-form double, compact plant, smaller flowers than most japonicas.
Camellia japonica ‘Mathotiana’Camellia japonica › · Historic deep crimson-red formal double, one of the longest-cultivated japonica varieties.
Camellia sasanqua ‘Narumigata’Camellia sasanqua › · Single, pink-tinged white, fragrant — one of the most popular UK sasanquas.
Camellia sasanqua ‘Hugh Evans’Camellia sasanqua › · Single rose-pink, fragrant, flowers over an unusually long late-autumn to late-winter season, sun-tolerant.
Camellia sasanqua ‘Plantation Pink’Camellia sasanqua › · Large scented pink flowers; RHS specifically notes it still needs shelter from cold wind and early morning frost, unlike some other sasanquas.
Camellia sasanqua ‘Setsugekka’Camellia sasanqua › · Pure white semi-double with frilled petals and yellow stamens, gently scented, one of the longest-flowering sasanquas.