In my experience, "telltale" and "spill the beans" are not synonymous. Usually the term "telltale" is used like in the following example:
"Frost on the ground is a telltale sign that it's cold outside."
It's something that is an indicator of something else.
Spilling the beans is to tell someone the whole story of a situation (when maybe some details are meant to be private).
Something similar (in spelling) to "telltale" that is more synonymous with "spilling the beans" is "tattletale". That's someone that purposefully goes and tells someone something private (like when a sister goes to her mom to tell her about something her brother did).
I hope this helps!
(edited to provide quotation marks above)