It is possible to see the last post made by a user. Click / tap on their avatar (picture) then scroll down to Activity. The first entry will be the most recent post.
It is possible to find users using the search bubble on wide displays, i.e. it doesn't work on narrow mobile displays. Tap / click on the search bubble, tap on the grey word (often Everywhere, but could be something else like This thread), and pick Members from the drop down list, enter as much of the username as you can, press enter. If an appropriate username is displayed, tap / click on Find activity.