The official name of the DLR on train crew is Passenger Service Agents (PSAs). Their primary purpose is safety related (they can drive the train if the automation fails and just generally keep an eye on things-they may have other rolls as well that I don’t know).
They also check tickets (if not doing anything more pressing), but are not authorised to issue penalty fares/reports for prosecution as far as I know. It is still useful to build data (and if someone touches a card on their reader that has not been tapped in, they will get a maximum fare and possibly blocked card). Occasionally, there are RPI (revenue protection inspector) station blocks/on train checks. RPIs can issue penalty fares/reports for prosecution and often have British Transport Police back up. Also, there are teams of undercover inspectors who track high level/repeat offenders (using data gathered from on the ground staff, dodgy card usage and CCTV) based on their regular travel patterns to catch them in the act. Basically, you are likely to get away with casual evasion for a while, but if you keep it up for too long are likely to come to grief some way or another.
Channel 5 made a series about TFL (and other) revenue protection-Fare dodgers at war with law. It is an interesting watch for those interested in the revenue side of things (though a bit over dramatised and full of recaps and “previews” rather than new content).