[Discovered this via a comment on another answer.]
According to their Wikipedia article:
Lecher lines were used as frequency measuring devices until frequency counters became available after World War 2.
The idea is to short a transmission line after some distance, forming what we now would call a resonant stub. The resonant reflections set up standing waves; the nodes of those waves could then be detected with an RF probe:
The voltage goes to zero at nodes located at multiples of half a wavelength from the end, with maxima called antinodes located midway between the nodes. Therefore, the wavelength λ can be determined by finding the location of two successive nodes (or antinodes) and measuring the distance between them, and multiplying by two.
But in practice, rather than having a fixed distance-to-short and moving an RF current probe, it sounds like usually there was a movable "shorting bar" under which was marked an appropriate scale:
In operation, the U end acts as a coupling link and is held near the transmitter's tank coil, and the shorting bar is slid out along the arms until the transmitter's plate current dips, indicating the first node has been reached. Then the distance from the end of the link to the shorting bar is a half-wavelength.
Apparently Lecher lines were even used to calculate the speed of light (working from known frequencies back…) before the turn of the century, "an important confirmation of James Clerk Maxwell's theory that light was an electromagnetic wave like radio waves"!
By September 1946, a "Frequency measurement at UHF" article in Radio News still listed this technique, as well as others which have already been mentioned in other answers/comments here:
[…] in general practice such great accuracy is not necessary, and frequency measurements at u.h.f. are generally performed by the following methods:
- Lecher wires.
- Heterodyne frequency meters.
IIUC, what the article describes as a wavemeter is essentially a resonant circuit with a conveniently-labelled variable capacitor.
(A PDF of the full issue is currently linked as the first citation on the Lecher lines article, which starts on page 50.)