predestined to be conformed to His image, i.e. to be one with Him, those He foreknew to love him, per v.28 , "And we know God works all things together for who love Him, who are called according to His purpose (which is to become one with Him).
This helps me answer the OP's question. You can tell that a person is elect if that person loves Him, even if they have yet to 'believe' in Him (i.e. they might hold a skeptical 'too good to be true' attitude toward him, initially, but, because of that love, or the love of the reputation that follows His person, keep searching Him out, and we know that scripture tells us that He is a rewarder of those that diligently seek Him...and ultimately find, indeed, to their delight, that He first loved them).