It would depend on the desease...but usually you cannot tell until after you have started the plants...for instance Anthracnose on tomatoes does not show up until after the fruit set and are growing large...and viruses take time to show also...
edited to say btw Anthracnose is a B**** to get out of the soil...and can spread to strawberries for one thing
Most things that are transmitted by seed are also near impossible to eradicate from the soil once you have them.
But, you know what, people survive anyhow
If you are concerned about a particular case -- and I can't think of too awful many situations where serious disease is a common problem through seeds, fusarium wilt in basil is the one that comes first to mind although there are others -- there is generally someone selling certified "clean" seed, so you could buy from those suppliers to the extent you worry.
Really the vast majority of garden plants' commercial seeds are AFAIK 'clean' with respect to anything that particularly matters.