dvd recorder/vcr combo

I have a GO VIDEO and MAGNAVOX combo unit. Both have tuners although both are useless. I need to use an external tuner for both. Both have an RF IN and OUT. The GO units RF OUT is an active modulator as all VCRs were. The MAG unit is only a pass through. I need an external modulator to send the output to the TV via an RF coax. My tvs do not have line in/out, analog. This being the case is there a way to tell by visual clues only whether a given unit uses an active or passive RF? Owners manuals for a used unit are never to be found. I have a manual for everything I own. Only the manual will tell me. I know this also applies to dvd recorders. I have never seen a dvd player with an RF. Those units are always connected to a vcr/dvd recorder/ tv directly. And even then a mod might be needed. Newer tech is always NOT better then what it replaced. Any insight welcome.