I have a deck I am working on that I cannot find all the cards for yet. For use in casual play I have put in proxies that are land cards with the name of the card written on them. If I end up playing with the deck in an official tournament and forget to switch the proxies out will I still be able to use those as lands? Obviously they cannot be used as the card they are a proxy for, but does having them marked affect their legality as the card they actually are?
You can use those cards as lands, as long as they are clearly those basic lands. The tournament rules on acceptable cards says
Artistic modifications are acceptable in sanctioned tournaments, provided that the modifications do not make the card art unrecognizable, contain substantial strategic advice, or contain offensive images. Artistic modifications also may not obstruct or change the mana cost or name of the card.
However, that rule also says
The Head Judge is the final authority on acceptable cards for a tournament.
So, it's up to the Head Judge to make the final decision about whether you can use those cards as lands.
You cannot use them as the cards they are proxying in a tournament. The official proxy tournament policy says
A proxy card is used during competition to represent an Authorized Game Card that has been accidentally damaged or excessively worn in the current tournament (including damaged or misprinted Limited product) as determined solely by the Head Judge.
Players may not create their own proxies; they may only be created by the Head Judge.
Basically, you can't even use a proxy card in your deck unless it is replacing a card that was accidentally damaged during that tournament, and you can't use proxies that you make yourself.
You are probably ok to use those lands legally in a tournament.
For marked cards:
"A card or sleeve is considered marked if it bears something that makes it possible to identify the card without seeing its face, including scratches, discoloration, and bends."
So any writing on a basic-land (not on the back) would be fine as long as it is not excessive. And even then, you are fine for basic lands because, "If a player is required to replace a card in his or her deck and is unable to find a replacement, the player may replace the card with a basic land card of his or her choice." This means you will never be hurting for basic lands.