Occasionally, maybe once per paragraph, misspell a word intensionally. Your family, knowing how carefully you used to profread your own writing, will notice this as abnormal behavior. Either you captors have already damaged your menial health or you are trying to conceal a message. Gards reading you letters before posting them may be more used to bad writing among their detaines and not suspect anything deeper. Your family might reply using the same code, both acknowledging receipt of your coded message, and perhaps including a key for a more secure one.
Occasionally, maybe once per paragraph, misspell a word intensionally. Your family, knowing how carefully you used to profread your own writing, will notice this as abnormal behavior. Either you captors have already damaged your menial health or you are trying to conceal a message. Gards reading you letters before posting them may be more used to bad writing among their detaines and not suspect anything deeper. Your family might reply using the same code, both acknowledging receipt of your coded message, and perhaps including a key for a more secure one.