I'd look at the IF stages first. Tuners are comparatively bulletproof in my (limited) experience. Make sure the tuner tubes are good and its contacts are clean.
Yes, there are ways to substitute for the tuner. Back in the day, they sold "tuner subbers," little gizmos that do what they sound like. You can also inject an IF signal (and do many other tricks) with a BK 1077 TV Analyst like this one:
http://antiqueradio.org/BK1077BTelevisionAnalyst.htm
You can also make simpler measurements such as checking the gain of each stage, checking voltages on each tube, etc. If you have a voltage chart, that's one easy way to start.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html