Matching impedance to grids with multi-tap driver transformer

[K1YTG]

I have a line to p-p grids transformer.  But no chart for the connections. 
Is there an in circuit way to try the different taps and measure which one gives the best match?
I have been thinking that the best match would be the one that delivers the most power the the grids.
So if I watch the output of the driven tubes (100th in this case),  the connections that give the most output would be the best match.
Or do I need to measure the grid current and the most grid current would be the best match?
Or is this all wrong?  Perhaps what I need to be looking for is lowest distortion.  I could watch the output on a scope and match for lowest distortion.
But I am not sure I could see the distortion on the scope.  Would I see any difference on the different taps? 
Thanks for assistance ,  Norm

[AC5UP]

If the transformer has any markings as to manufacturer or model number it might be possible to find the application data on the web. There are web sites with vintage catalogs from Thordarson, Stancor, United, Chicago Transformer and others.

I had a mystery transformer with only a model number that I was able to trace back to Stancor by looking at the page indexes of catalogs  HERE  and  HERE.

Some of the old catalogs also include application notes, sample circuits and tips for selection and use of their products which could be useful even if you don't find the exact part. There is some consistency in color codes and pinouts within transformer types across manufacturers.