That's really tough to tell, what @bstpierre commented is correct, but also difficult to compute. So I went out and tried to find popular seeds that are often planted in containers. Hopefully they're one of the following. You can maybe cut out the cherry's and yellow's assuming that's not what the box looks like.
These tomatoes are popular for use in containers, hanging baskets and garden or patio locations where space is limited. Because more people now live where traditional vegetable gardening is not possible, container and patio gardens have become more popular. Their ornamental value is an added benefit and their fruit quality has recently been improved as well. They have fruit in red and some other colors and are not suitable for pruning (except the new Husky hybrids).
Tiny Tim (45 days to harvest; 1 inch; very dwarf, red cherry fruit; determinate)
Cherry Gold (45 days; 1 inch; golden version of Tiny Tim; determinate)
Red Robin (55 days; 1 inch; super-dwarf plant, 6 inches tall; mild taste; determinate)
Yellow Canary (55 days; 1 inch; similar to Red Robin, but yellow fruit; determinate)
Pixie Hybrid II (52 days; 2 ounces; compact dwarf plants; determinate)
Patio Hybrid (65 days; 3 ounces; strong dwarf plants, relatively large fruit, ideal container plant; determinate)
Small Fry (72 days; 1 inch; red, good in hanging baskets; determinate)
Husky Red Hybrid (68 days; 6 ounces; dwarf plant, large fruit; extended harvest; indeterminate; resistant to VF)
Husky Gold Hybrid (70 days; 6 ounces; AAS winner; same plant types as Red and Pink; gold fruit; indeterminate; resistant to VF)
Husky Pink Hybrid (72 days; 6 ounces; smooth pink fruit on same husky-type plant; indeterminate; resistant to VF)
So, if you have some recollection of what the seed looks like, and it looks like this:
Husky Hybrid Seeds
then you've probably got an indeterminate tomato. Otherwise, it's determinate.