...(I think you meant 139XLR)...
Well, you can do a simple ohmmeter check (continuity) with the power off and just locate foil ground...test the ohmmeter at 1X setting and find the lowest STEADY ohmic reading and that is your Caps "banded Cathode" side.
To help with most of the Electrolytic caps you're not sure how to install or orient correctly - find a shielded can case - RF cans work, and put one lead to it, and the other lead of the DVM to the caps "holes" - whichever one shows nearly dead short less than 2 ohms, that should be negative - the other hole would exhibit a ohmic reading that wildly changes.
And if you're in a spot where you need to know polarity of a cap you found bad, you can remove the cap and apply power to the board, and measure the voltage DIFFERENCE in the holes the cap goes. The side with the highest Positive value compared to negative - is where the UNBANDED or "+" mark (if tantalum) goes - this keeps the cap's plates properly set so it doesn't blow or cause excessive gassing and eventual failure.
If you need more help, CBRadio Magazine did up a short You Tube vid introducing you to the 139XLR, MikesRadioRepair did another - you can review these to get clues on how to proceed with the work you want to do or repair.
In light of your situation - the C304 cap the Banded end (Cathode) is towards the front of the Radio - right by the speaker - so when you install the new one it's banded end is right behind the speaker.
You commented about "great carrier but low modulation" Locate VR 8 - by the rear modulator speaker coil core - if you can adjust the carrier, see if turning down the carrier restores the modulation level - any carrier above 5 watts this radio tends to pinch audio up and nearly off.
Else more work around the Final and Driver to help with the RF tune up may just be all it needs.