To expand a bit then yeah two approaches
1) Somewhere in the game (possibly monster stats table) will be how much experience and gold drop there is. Go down the list doubling things and you have your hack. Bit tedious but gets the job done, and can allow some finer grain control (not to mention if there are drop rates for things then you usually have them in the same tables) if you would rather not have certain enemies/sections benefit your play too much.
2) Edit the after battle results in memory. This is what most cheats on later systems that do the whole 0.5x,2x,4x,8x,16x... gold/experience bit will do. The multiplying by two also speaking to how they tend to do it, that being shifts* which the simpler cheat options most NES devices and emulators go in for will probably lack (they tend to be more raw memory reads and maybe an IF ELSE). That said if doing it as a ROM hack you probably can find a way for it to add up everything for the individual monsters (assuming it does that rather than have a fixed value, per level, by location...) and then chucking in a nice little shift (6502 has options for both left and right shifts in case you decided you want a 0.5x "hard mode") before it displays and adds it to your experience/gold pool.
2) obviously will be assembly but probably one of the easier options here -- do a normal cheat search to find the gold and exp locations. Set a break on write for those and win a battle. When it writes it then the emulator will flash up and say something wrote it and what did it as well as what the previous however many instructions leading to that were. Follow it back up until you get a suitable point (might have to do another break if you have to go really far back to get to before the on screen results are shown so you can have them be "correct" but no big deal) and add in a shift (or more likely a jump to somewhere else, the shift and whatever you broke by adding a jump before jumping back, or if you can combine both gold and exp into one setup then do that).
*in case you were unfamiliar. Every schoolboy will tell you multiplying by 10 is easy as you just add a 0 on the end, though the better ones will tell you you shift the decimal point around. Computers not using decimal but binary meaning if you shift the data around you multiply it not by 10 but by 2 (binary is called base 2 after all).
For some games there will be a third option. If there is a mode where you gain double exp (winning a battle with no damage, surprise attack, simple random bonus option, because of an item you have, because it is a flashback sequence, levelling catchup mechanic... plenty of games have different things but the same end result) and/or gold then there might well be a flag or multiplier somewhere in the mix for that. Find it and edit that by cheat in the case of a flag or forcing it to take the max bonus path. I tend not to see this for NES games though, even 16 bit era this is rare for and is something I tend to associate more with PS1 on up to today (as well as contemporary handhelds to that).