The odds are good that the place where you purchase the replacement antenna will also sell SWR meters. If not, the closest major truck stop surely will, e.g., Pilot, TCA, Love's, etc.
What will happen if you just use the new antenna untuned? Probably nothing unless it is way off. There will be no ill effects on your radio during Rx. Reflected energy from an untuned antenna during transmit is what potentially causes damage. Keep your transmissions short until you can tune the antenna and hope for the best.