FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 197118
This article describes how to verify Radius server user credentials via GUI/web interface of the FortiGate.

Before FortiOS 6.0.0, it was only possible to check the Radius user credentials via CLI. 
However, starting from FortiOS 6.0.0 and onward, this feature is available on the GUI as well.

In order to check the Radius server user credentials, go to User & Device -> Radius servers
Edit the configured Radius Server and click to the “Test User Credentials” button.

Note: Before testing user credentials, make sure that the Radius Server is already configured and there is no connectivity issues between the FortiGate and Radius Server.

After that, enter the username and password of the user.

As it is shown on the above screenshot, the FortiGate can check “Connection status” and “User credentials” and in case of successfully authenticating, it shows additional information such as  Radius “AVP” and “VSA”.

Note: In current FortiOS implementations, the Radius server user credentials via GUI/web interface supports and works only with the PAP (Password authentication protocol) scheme.
If the user credentials is tested with the Radius Server that does not have “PAP” enabled, the FortiGate will show “Invalid credentials” message:

For all schemes than “PAP” it is recommended to test it via CLI:
# diagnose  test  authserver  radius
<server_name> <chap | pap | mschap | mschap2> <username> <password>
For example:
# diagnose  test  authserver  radius WIN16 mschap2 radiususer1 P@$$w0rd1
authenticate 'radiususer1' against 'mschap2' succeeded, server=primary assigned_rad_session_id=457812022 session_timeout=0 secs idle_timeout=0 secs!
Group membership(s) - radiusgroup

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