...a easy way to check coax i found is a 50 dummy load at one end, and a ant anilizer like the mfj 259. see what the resistance is.
If you put a dummy load on the far end and check the SWR (or RF resistance), even a lossy feedline
will still look like 50 ohms. That is NOT a good check for losses unless you use a power meter at both
ends. A low SWR in a system is often an indicator of HIGH losses in the coax.
Instead, use Peter G3RZP's approach: disconnect the antenna from the far end of the coax (or use
a band where you expect the SWR to be high) and check the SWR. The higher the SWR, the lower
the loss of the cable. Because the far end is disconnected (or shorted, if that is easier) the SWR
at the antenna is high, and it should be high also at the rig end unless the cable is lossy. If the
SWR is 3 : 1 at the rig (or better) in that case, consider replacing the coax.
I use this with my triband yagi: I check the SWR on 12m or 17m, and if it is too low then the
coax is too lossy. Using a band where the SWR is high makes it easier because I don't have
to climb the tower to disconnect the coax, especially if I were smart enough to check the SWR
on all bands when I first put up the antenna, then checking it periodically will tell me if the
coax has degraded.