Checking the CAPS LOCK state at startup of an application (e.g. an password-entry) can be done with a little help from a small wonderful commandline-tool: xdotool.
In the example below I let xdotool fire a minor 'x' to see what character will be received by the previous focussed label-widget. In case of a major 'X' the CAPS LOCK key had been pressed obviously and a corresponding message will be displayed.
proc main {} {
wm resizable . 0 0
label .message -text {CAPS LOCK disabled} -textvariable MESSAGE -width 20
bind .message <KeyPress> {if {%N eq 88} {set ::MESSAGE {CAPS LOCK enabled}}}
pack .message
focus .message
update
catch {exec xdotool key x}
after 3000 exit
}main