The thingSpeak communication library function ThingSpeak.writeField will return the HTTP status 200 if the transaction was successful. See the example for Arduino prototyping in the examples section. You can use a separate function for writing such as the one below. Then output the value of writeSuccess that is returned by this function to your serial monitor. If returns 200 or 202, you are good.
int writeTSData(long TSChannel,unsigned int TSField,float data,char* ReadAPIKey){
int writeSuccess = ThingSpeak.writeField(TSChannel, TSField, data, writeAPIKey); //write the data to the channel
return writeSuccess;
}
What library are you using for your wifi connection?