Yeah, it's a tricky when the plural form is the same as the singular, isn't it? I think there are are probably even situations where you just can't tell which is which unless there's more surrounding text to give you additional context. However, in this particular case, I think it would have read "Deine Schlüssel" if it had been plural.

See the "Possessive pronouns" table on the following Wikipedia page to see what endings are slapped on to the root form of the possessive pronoun for the various genders and cases.

http://en.wikipedia.org/wiki/German_pronouns

Dein is singular. If there were more than one key, it would be Deine. The word Schluessel is the same whether singular or plural.