You can always use a knife to cut out the seed, but lychees should be eaten whole. The skin should be shiny and bright red. If the fruit turns brown or grey, it is spoiled or rotten. However, if it is still green or yellow, it can still be consumed. If the skin is dark, it is best to throw it away and compost it.
Look at the inside of the lychee. The flesh should be white and firm. If it is discolored, it is likely fermented and must not be eaten.