Long listening sessions are a good method. However, if A/B switching is desired too, then use two turntables to do quick switch A/B listening. Many of us have more than one turntable.
Of course the tables will not sound the same, but differences can be noted from the A/B listening, then swap the turntables into the phono preamps and listen to what differences switch or persist. This would help extract differences attributable to the turntables vs the phono preamps.