As far as I know, there is no diacritic for this in common use. Much like how there's no diacritic to mark a silent "E" in English—readers are just supposed to get used to the situations in which the letter is silent (word-finally after a consonant) and the exceptions where this doesn't apply (like "cafe").
But if you want to come up with one for your own use, perhaps to use on flash cards or whatever as a mnemonic, there are several Arabic-script diacritics represented in Unicode which aren't used in standard Persian. The sukūn (which indicates a lack of a following vowel in Arabic) is probably the easiest and most distinctive; it looks like a small circle above the letter (so, "خوْاب").
If this helps you learn the words, by all means go for it! Just be aware that it isn't a standard practice. It's somewhat like writing English silent E as "ĕ"—native English-speakers will still understand what you mean, but will find the new diacritic strange and unusual, and probably have questions for the author about it. So I wouldn't recommend it outside study materials for your own use.