CheckTx is called by the BaseApp when comet receives a transaction from a client, over the p2p network or RPC. The CheckTx method is responsible for validating the transaction and returning an error if the transaction is invalid. baseapp/abci.go 
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See full example on GitHub

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CheckTx Handler​

CheckTxHandler allows users to extend the logic of CheckTx. CheckTxHandler is called by passing context and the transaction bytes received through ABCI. It is required that the handler returns deterministic results given the same transaction bytes.
we return the raw decoded transaction here to avoid decoding it twice.
type CheckTxHandler func(ctx sdk.Context, tx []byte) (Tx, error)
Setting a custom CheckTxHandler is optional. It can be done from your app.go file: 
func NewSimApp(    logger log.Logger,    db corestore.KVStoreWithBatch,    traceStore io.Writer,    loadLatest bool,    appOpts servertypes.AppOptions,    baseAppOptions ...func(*baseapp.BaseApp),) *SimApp {  ...  // Create ChecktxHandler  checktxHandler := abci.NewCustomCheckTxHandler(...)  app.SetCheckTxHandler(checktxHandler)  ...}