In circuit resistor testing?

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On carbon comp resistors, a gold last band is +/-5%, a sliver last band is +/-10%, and no last band is +/-20%

Generally, checking resistors in circuit is not possible. To some extent you can, if you follow the schematic while doing it. If there is no other DC current path across the resistor, it should measure normal. if it measures too high (out of original tolerance), that generally means it's bad, or at least lift one lead and try it again to make sure you really saw what you thought you did. If there IS another DC current path, you have to lift a lead. Theres no other way.

You can troubleshoot most problems by checking DC voltages. Start with the power supply. When you look at the schematic (if its a sams anyway) there will be DC voltages listed at several power supply taps, and on the elements of most tubes. You are looking for stuff that is way off.