There are not too may ways to check the tube without an expencive tube tester but some sympthoms may tell you that the tube may be bad.
First, at a glance, usually the bad one has black residue on the glass bulb. Second is to compare with good one and checking the bias behaviour: If the bias goes up fast over the limit shut-off the amp and here you spotted the bad one and contrary if the bias doesn't go up at all than you've probably got the bad one.
For contact enhancement you can use Pepsi that can also be a good solution to get rid of corrosion as well. After such experiment with Pepsi, you may probably not want to drink it thereafter:-).
First, at a glance, usually the bad one has black residue on the glass bulb. Second is to compare with good one and checking the bias behaviour: If the bias goes up fast over the limit shut-off the amp and here you spotted the bad one and contrary if the bias doesn't go up at all than you've probably got the bad one.
For contact enhancement you can use Pepsi that can also be a good solution to get rid of corrosion as well. After such experiment with Pepsi, you may probably not want to drink it thereafter:-).