There isn't an exact way to know how many EVs your Pokemon has in gen 3.

However, you can increase EVs by 10 each using the EV increasing items that can be bought at the celadon department store. (These are very expensive so it would take some time to get an adequate amount)

Then to estimate how many EVs a Pokemon has, save then feed the Pokemon the vitamin for the stat you want to find the EVs of until it cant be fed any more, meaning either that stat's EVs are max or you have the maximum of 510 evs on that Pokemon. Count how many vitamins it took to get it to max and multiply that number by 10, then subtract that number from 255. The number you get should be a rough estimate of the amount of EVs that Pokemon has in that stat. Now you can reset to the save if you want to check another Pokemon/stat's EVs.

The EV-reducing Berries do not work in FireRed or LeafGreen, so it's a good idea to keep track of the Pokemon you fight.

Hope this helps.