An each-way bet splits your stake: half on the outright win, half on a 'place' (finishing in a defined set of positions) at a fraction of the win odds. In football it appears on outrights — top-4 finish as the place on a title bet, for example.
The value question is always the place fraction: at a quarter of odds of 8.00, the place pays 2.00 against a top-4 probability the market may price closer to 1.60 or 2.60. The two halves are one decision only in the slip — evaluate them as two separate bets, because that is what they are.
£10 each-way (total £20) on a team at 8.00 for the title, place ¼ odds top-4. Title miss but top-4 hit: the win half loses £10, the place half returns £10 × (1 + 7/4) = £27.50 — net +£7.50. Whether that place price beat its true chance is the actual value question.