Nutrients in compost

Seedlings tend not to need any food (seed compost has no / minimal nutrients)

Multi purpose compost fertilizer usually lasts about 5 weeks - by which time the plants will usually be potted on, and thus continue to not need feeding.

Most people don't feed Tomatoes until they start flowering. Other things that have not been potted on for 6 weeks will probably need feeding.  I wouldn't worry about it too much - give a very weak feed if you aren't sure, and look out for signs of yellowing or lack of a lush green, in the leaves.