Can I check the "in-travel" status of my Oyster card?

Travel history is made available to those who register their Oyster Card online, and only becomes available the following day. So you won't be able to check when you are in the middle of a journey. If you haven't registered your card, you can still view the last 7 days history.

You can check the online journey history for previous days and look for Incomplete journeys.

More on Incomplete Journeys here:

• Incomplete Journeys
• If you don't touch in or out

If you don't touch in and out

If you're using pay as you go on Tube, DLR, London Overground, TfL Rail or National Rail services in London and don't touch in or out, you may have an incomplete journey and be charged a maximum fare of up to £8.80.

If you have an incomplete journey we don't know where you started or ended your journey meaning we are unable to charge you the right fare.

When you touch in at the start of a Tube, DLR, London Overground, TfL Rail or National Rail journey in London, we deduct up to £8.80 from your pay as you go credit.

When you touch out at the end, we work out the right fare for your journey and the amount deducted from your credit is adjusted.

If you don't touch in and out correctly, we don't know what to charge you, so a maximum fare of up to £8.80 is deducted from your credit.

Maximum fares don't count towards capping and don't apply on buses and trams.

When stations are very busy we may leave the gates open to control crowds and ensure that people can leave the station quickly and safely. If so, you might not be able to touch out but we'll aim to fix this for you when you touch in to start your next journey.

P.S. Technically you need an address to register your Oyster Card, so Tourists are officially advised to use a Visitor Oyster Card.

P.P.S TFL is planning to replace Oyster with Contactless Payment Cards anyway, which according to @ecatmur you can check your Dashboard (requires registration) to get up to the minute updates.

(Note to readers: The original question is asked in mid-2015, and the answer was then no. Things have changed and now it is possible to check your touch-in/out history while still on the same journey.)

In late 2017, TfL launched the TfL Oyster and Contactless mobile app. The app contains multiple features, including:

• View your Oyster card and contactless journey history
• Check if you have incomplete journeys

In particular, in the latest terms and conditions at the time of writing, TfL mentioned the possibility of viewing journeys within 30 minutes of it being made (Section 3.2, emphasis mine):

Once you’ve signed into the App you can:

• [...]
• View your recent journey history for each registered Oyster or contactless card and what you were charged for your journeys. All journeys will usually be available to view within 30 minutes of being made.

This means once you've linked your Oyster/contactless card to your account and signed in to your app with that account, all recent journeys will show up on your mobile screens within 30 mins. In practice, this means all touch-in/outs.

Missing touch-in

The OP asked whether they can see if there is a journey in-progress/live, with the aim of detecting a potentially missing touch-in. This is only possible if their journey lasted for more than 30 minutes, and can be inferred (with some uncertainty) by a lack of new journey record when 30 minutes has elapsed. Alternatively, the OP will be able to tell they have forgotten to touch-in:

1. When touching-out, as the ticket barrier will charge a incomplete journey fare (assuming no travelcard is in play here); or
2. Within 30 minutes of touching-out, as the journey will have a red "Missing touch" message in place of the origin of their journey on the app.