If it's with someone who is ESL with limited vocabulary and never played Scrabble before with you being a native speaker who played Scrabble lots.
Start at 200 points. (To make sure they win the first game.) Every game they win, until you win, subtract 10 points from the handicap. After you win once, reduce handicap by 5 pts every time they win 3 (or 2, decide with your opponent) times running.
Hidden assumption - Opponent is a competitive Type-A personality who really wants to improve their vocabulary, and is also doing crosswords a few times a week.
I play Scrabble with kids all the time, I think the same concepts would work nicely with an ESL opponent. To me handicapping by points (opponent starts with 150 or 200 points or whatever) does nothing to make the game more fun for either player. What does work is allowing my young opponent all the time they need for each turn - I get timed. They get all of the good cheat sheets - I don't (that one is fair even even if I'm playing an adult native English speaker with less Scrabble experience). And if the gap is really big - especially if Scrabble is being used as an educational tool - allowing my opponent timed use of the dictionary or OSPD while figuring their play. For the last one, consider limiting the time the opponent gets to use the dictionary more and more as the gap narrows.
BTW, using timing the stronger player as a handicap only works if that player draws tiles at the beginning of their turn instead of the end of their last one.
- Give them more Tiles. I think this helps expand what they can do rather then just artificially boosting the score. And the goal of games is engagement not winning. (I know competitive people want to win, but games are about engagement)
- Let them pick their tiles at the END of their turn (so they get to look at them longer). Everyone else picks at the start of their turn. Gives them a little time advantage.
- They get credit for a BINGO if they use just 7 Tiles.
- Give them BINGO +1 credit for each tile beyond BINGO they use.
As others have mentioned, it's not merely a question of strategy and expertise - English spelling rules being what they are, your typical ESL player is probably going to take a lot of penalties for challenged words if you play strictly by the rules. I would think you'd want to not only give them a numeric handicap, but maybe soften up the rules on challenges a bit. Perhaps a successful challenge just means they need to try again with a different word, rather than making them lose their turn.
Similarly, the biggest difference between novice and intermediate Scrabble players is probably knowing the list of 2-letter English words. I have seen people play Scrabble with a little cheat-sheet of the two letter words as a way to level the playing field.
In certain games like golf, the handicap is set by "experience".
That is, each player will know their scoring history and compare them. The weaker player wins if s/he can beat the "spread" suggested by these histories. That may be the best way to compare and "handicap" two players of very disparate backgrounds.
Scoring will be frustrating for one player or another. If the handicap is too harsh, the native speaker will have a hard time; if the rules are too lenient, the ESL player will be frustrated... The best solution would be to play Scrabble just for fun. If you want to play competitively, Scrabble isn't the right game.