You can check the members of the variables viewoptions and visoptions in ngsolve.internal:
Code:
>>> import ngsolve.internal
importing NGSolve-6.2.1908-6-gf77e4e6b
>>> dir(ngsolve.internal.viewoptions)
['_attributes', '_name', '_update_cmd', 'centerpoint', 'centerx', 'centery', 'centerz', 'clipping', 'colormeshsize', 'drawbadels', 'drawcolorbar', 'drawcoordinatecross', 'drawcurveproj', 'drawcurveprojedge', 'drawdomainsurf', 'drawededgenrs', 'drawededges', 'drawedgenumbers', 'drawedges', 'drawedpointnrs', 'drawedpoints', 'drawedtangents', 'drawelement', 'drawelementnumbers', 'drawfacenumbers', 'drawfilledtrigs', 'drawhexes', 'drawidentified', 'drawmetispartition', 'drawnetgenlogo', 'drawoutline', 'drawpointnumbers', 'drawprisms', 'drawpyramids', 'drawspecpoint', 'drawtets', 'drawtetsdomain', 'light', 'mat', 'shrink', 'specpointvlen', 'specpointx', 'specpointy', 'specpointz', 'stereo', 'usecentercoords', 'whitebackground']
>>> dir(ngsolve.internal.visoptions)
['_attributes', '_name', '_update_cmd', 'autoredraw', 'autoredrawtime', 'autoscale', 'clipsolution', 'deformation', 'drawfieldlines', 'drawpointcurves', 'evaluate', 'evaluatefiledescriptions', 'evaluatefilenames', 'fieldlinesfilename', 'fieldlineslength', 'fieldlinesmaxpoints', 'fieldlinesonlyonephase', 'fieldlinesphase', 'fieldlinesrandomstart', 'fieldlinesrktype', 'fieldlinesstartarea', 'fieldlinesstartareap1x', 'fieldlinesstartareap1y', 'fieldlinesstartareap1z', 'fieldlinesstartareap2x', 'fieldlinesstartareap2y', 'fieldlinesstartareap2z', 'fieldlinesstartface', 'fieldlinesthickness', 'fieldlinestolerance', 'fieldlinesvecfunction', 'gridsize', 'imaginary', 'invcolor', 'isolines', 'isosurf', 'lineartexture', 'lineplotautoscale', 'lineplotcolor', 'lineplotcurrentnum', 'lineplotdatadescr', 'lineplotfile', 'lineplotinfos', 'lineplotselected', 'lineplotselectedeval', 'lineplotselector', 'lineplotsizex', 'lineplotsizey', 'lineplotsource', 'lineplotusingx', 'lineplotusingy', 'lineplotxcoordselector', 'lineplotxmax', 'lineplotxmin', 'lineplotycoordselector', 'lineplotymax', 'lineplotymin', 'logscale', 'mmaxval', 'mminval', 'multidimcomponent', 'numfieldlines', 'numiso', 'numtexturecols', 'redrawperiodic', 'scaledeform1', 'scaledeform2', 'scalfunction', 'showclipsolution', 'showsurfacesolution', 'simulationtime', 'subdivisions', 'usetexture', 'vecfunction', 'xoffset', 'yoffset']
Note that, due to asynchronous evaluation, there is only write and no read access to these values.