I don't know if this is the official "right" answer, but here's what I do. . .
2 or 3 hours after I take anything out of the canner, I check the seal by gently touching the middle of the lid. If there is any give, I know that that jar has not sealed. So I take my sharpie and put a little dot on the lid. The next night (I do all my canning in the evening), when I take off my rings and wash my jars, any jar that has a dot on the lid, I check those jars extra carefully for the seal. Anecdotaly I have found that if a jar doesn't seal within an hour or 2, most likely it isn't going to seal. Based on the system I use, I would feel comfortable letting my jars sit an extra day or 2 before removing rings, washing and putting away for storage.

Thank you Carol and Pixie Lou. This is helpful!

When I find a jar that looks like it doesn't want to seal after about 5 minutes, I pour boiling water over the lid. This sometimes convinces it to pop.

5 mins. isn't nearly long enough to wait for all the air to be evacuated from the jar so you are creating a weak artificial seal, not a safe one. If a lid doesn't form a true seal within 10-12 hours then there is something wrong that needs to be fixed and the jar reprocessed or refrigerated.

Dave

The strength of the seal can be checked once the jars have cooled. What I find is that if the other jars in the batch have popped, the boiling water is a test. If it doesn't work, then I figure without waiting that there is something wrong with the seal and I will have to reprocess the jar.

how long in the fridge can you leave the jar that didn't seal?

Treat your unsealed jar of tomatoes like you would any other unsealed food item that you have in the fridge. I'd probably use it up within the week and make a batch of spaghetti or stew or soup.