Garden centers normally have that information on the transplant label and catalogs usually include it in the variety description. If neither of those fits your case, the plants will give you some hints. Determinate tomatoes are more compact, stocky, upright and have more densely-packed leaves. The older plants don't sprawl and thus don't need quite as much staking to keep them upright. Most tomato varieties are indeterminate - that's the default - plants are taller, leggier and the foliage is more sparse. They will sprawl across the ground if you let them. But it's not too hard to tell with young plants -do they look more tall and spindly or short and compact. Some varieties are called semi-determinate, which complicates matters a bit, but not enough to matter.