Listen to reactive() invalidation in shiny?

is there an exposed (maybe even non-exposed) function in shiny that would tell me when a reactive got invalidated? Draft of example:

ui.R

selectInput('select', ...)

server.R

myfunc <- reactive({ ... input$select ... })

output$listen <- textOutput({
   valid_flag <- ???(myfunc)
   paste('This function is', valid_flag)
})

The ???() function would give me the information if the reactive is valid or invalidated.