if you meant to ask:

How can I use radio to determine if line-of-sight exists at a distance beyond what the naked eye can see?

then I would say:

Using a frequency of something with sufficiently narrow beam-width, yes, you can determine if pracical line-of-sight exists between two points.

A laser may be sufficiently narrow... but it may also be too narrow in that the aiming precision required may give the impression that you don't have line of sight when, in fact, you just aren't able to suitably control targeting of the transmitter and receiver.

Microwave dishes, UHF yagi arrays, or even just a single UHF yagi may be 1) of sufficient beam-width to make aiming feasible and 2) of sufficient narrowness for the purpose.

The actual distance between your 2 points will matter, as will atmospherics (temperature, humidity, precipitation, etc) and other environmental factors.

if you meant to ask:

How can I use radio to determine if line-of-sight exists at a distance beyond what the naked eye can see?

then I would say:

Using a frequency of something with sufficiently narrow beam-width, yes, you can determine if pracical line-of-sight exists between two points.

A laser may be sufficiently narrow... but it may also be too narrow in that the aiming precision required may give the impression that you don't have line of sight when, in fact, you just aren't able to suitably control targeting of the transmitter and receiver.

Microwave dishes, UHF yagi arrays, or even just a single UHF yagi may be 1) of sufficient beam-width to make aiming feasible and 2) of sufficient narrowness for the purpose.

Given modern ability to measure precise location with GPS and calculate relevant antenna bearings, you might even look into using computer calibrated and controlled antennas to accomplish this.

The actual distance between your 2 points will matter, as will atmospherics (temperature, humidity, precipitation, etc) and other environmental factors.

In theory: no. Practically: yes, for a while.

Theory

Suppose a modulated perfect laser is used for communication. If I were a dedicated listener, I would erect a half-silvered mirror at a 45º angle to the laser to capture part of the signal so that I can listen in.

It's possible that the receiving end can detect my "stealing" part of the signal since there would be a resultant reduction in signal intensity... but detection is different from prevention.

Practice

Yeah, nobody's erecting a half-silvered mirror to do this. And you don't have access to a modulated perfect laser.

You can setup a dish based microwave link that's close enough depending on requirements.

Regardless, this is an example of security through obscurity. It will last for a finite amount of time until adversaries find out, and decide that they care to intercept/listen in.

Whether the finite time is acceptable to you depends on other factors :-)

This is why encryption exists (NB it's not kosher in amateur radio)

Encrypting a signal can obviate the need to hide it, and is why it exists. Once a signal leaves your radiator (laser or otherwise) there's no guarantee that it will stop at the border of the receiver... it will likely BOTH keep going in the original direction AND be partially reflected at some angle(s).

if you meant to ask:

How can I use radio to determine if line-of-sight exists at a distance beyond what the naked eye can see?

then I would say:

Using a frequency of something with sufficiently narrow beam-width, yes, you can determine if pracical line-of-sight exists between two points.

A laser may be sufficiently narrow... but it may also be too narrow in that the aiming precision required may give the impression that you don't have line of sight when, in fact, you just aren't able to suitably control targeting of the transmitter and receiver.

Microwave dishes, UHF yagi arrays, or even just a single UHF yagi may be 1) of sufficient beam-width to make aiming feasible and 2) of sufficient narrowness for the purpose.

The actual distance between your 2 points will matter, as will atmospherics (temperature, humidity, precipitation, etc) and other environmental factors.