You only need to be concerned with cross pollination if you're planning on saving seeds from this season's crop. This season, the Amish Paste will give you Amish Paste and San Marzano will give you San Marzano.

Cross Pollinating faq

And even if you are saving seed the odds of crossing are minimal. If you want a guarantee of 100% purity then just bag some blooms.

You might be able to ID the slightly wispier foliage of the Amish Paste and it is a much smaller plant but otherwise you'll have to wait for fruit to form as they have very different fruit shapes.

Dave

Yeah I was,afraid I would have,to wait until after they fruited which would be too late :(

Thanks forcthe,help though!

As Dave mentioned the Amish paste might display a wispier collage. Also the San Marzano growth habit is semi-determinate so plant will not grow as upright as the Indeterminate Amish Paste plants. You probably won't notice these differences for weeks but ultimate plant height and deciding on a support method would be my greater concern.