How can I improve my chances at probing minds?

Question

Sooner or later my warlock will need to probe minds again with Detect Thoughts and will end up matching wits with a Sicilian the target in a contested intelligence check.

Either way, the target knows that you are probing into its mind, and unless you shift your attention to another creature's thoughts, the creature can use its action on its turn to make an Intelligence check contested by your Intelligence check; if it succeeds, the spell ends.
— Detect Thoughts

My Pact of the Tome Warlock, her patron is a Great Old One, only has a 12 intelligence, and we play a low magic item game (we haven't had any at level 4 — it's low combat so that hasn't been an issue). I have concentration spells that would help (i.e. Cause Fear, Hex, Guidance), but I can't use as Detect Thoughts is also concentration. I have just learnt Find Familiar, but I'd want to check with my DM before assuming the help action from the familiar would be allowed.

How can increase my chances to win the check? Preference is given to solo solutions; roleplay-wise my character's warlock abilities are being treated as something the other characters don't want to get too involved in.

Answer 1

To increase your own check:

I'm limiting magic items to uncommon rarity, because it's been mentioned this is a low magic item game.

Without ally assistance:

• Grab the Lucky feat to reroll ability checks.
• Find a Stone of Good Luck (Luckstone) for +1 on all ability checks.
• Find a Headband of Intellect to increase your Intelligence to 19.

With ally assistance:

• Have an allied caster (Artificer, Bard, Cleric, Druid, or Sorcerer) cast Enhance Ability on you, granting you advantage on Intelligence checks with Fox's Cunning.
• Have an allied Bard grant you Bardic Inspiration.
• Have an allied Cleric or Druid cast the Guidance cantrip on you.

To decrease the target's check:

With ally assistance:

• Have another party member cast Hex on the target, granting disadvantage on Intelligence checks. Unfortunately, you cannot do this yourself because both Hex and Detect Thoughts require concentration. A non-Warlock member of the party can pick up Hex with the Magic Initiate or Fey-Touched feats.
• Wrap the target in metal chains and have an allied Druid, Bard, or Artificer cast Heat Metal on them. A creature that takes damage from heat metal has disadvantage on ability checks for a round, and the ally can continue dealing the damage to extend the effect. This method is a little extreme, but could give that intense interrogation flavor if you and your allies are ok with that.
• Have an allied Warlock or Wizard cast Cause Fear on the target and remain in their line of sight. The frightened status effect grants disadvantage on ability checks (and attack rolls) while the object of your fear is in your line of sight.
• Have an allied Paladin hit the target with a Wrathful Smite and remain in their line of sight. This spell also causes the frightened condition.

To prevent the target from making a check:

The target needs to use their action to make the contested check in the first place, so any effect that causes incapacitation will prevent them from initiating the contested check. Options available at your level include:

Without ally assistance:

• Cast Detect Thoughts on the target while they are sleeping naturally or after they have been knocked out in combat. An unconscious target is incapacitated and cannot take actions. You probably won't get any surface thoughts from an unconscious target, but you can probe deeper to learn "something that looms large in its mind", which may occupy its thoughts even while asleep. Ask your DM whether or not they will allow this; the spell does not mention how it interacts with a sleeping or unconscious creature.
• If the DM allows the above, the 1st level spell Sleep (not available to Warlocks, but you can gain it with the Magic Initiate or Fey Touched feats).

With ally assistance:

• The 1st level spell Tasha's Hideous Laughter (must be cast by an ally, because it requires concentration).
• The 2nd level spell Hold Person (must be cast by an ally, because it requires concentration)

Wildcard: convince the target to allow you into their mind.

Without ally assistance:

• Cast Charm Person on the target and convince them you're doing this for their own good. Then cast Detect Thoughts on them, once the target is charmed and believes you are a friend trying to help them in some way. Hopefully, they will not attempt to force you out with the contested Intelligence check if they think you are a friend or ally. Charm Person does not require concentration and is available to Warlocks, so you can try this yourself if you have a spell slot to burn. It may or may not work depending on the DM, the target, and the scenario.

As you and your party level up, you will gain more options to improve your own ability checks (multiclassing into 2 levels of Bard will allow you to add half your proficiency bonus to your Intelligence checks with the Jack of all Trades feature, for example), decrease an opponent's (the 5th level spell Synaptic Static, for example), or incapacitate a target (such as the 3rd level spell Hypnotic Pattern or a Monk's 5th level Stunning Strike feature). Keep an eye out for these to find other ways to improve your chances as your character progresses.