18+ months
Observation
Learn to speak the same language. You could either learn about your own toddler's specific habits first, with some observation time, and then try to bring some easy 'catches' into your day-to-day routines, or you could try to catch some pee or poo straight away by using some common knowledge, and then use observation time to get to know your toddler's normal rhythms, whether they are giving off any signals beforehand and to develop your sense of intuition. The key is to do both! Repeatedly observing and practicing will help you and your toddler learn together and build trust and confidence in each other. All the information you need to get started is described below.
Step 1: Watch your toddler
Observe your toddler during nappy-free time or while wearing cotton training pants, so you can see any wetness and they can feel it immediately. Try it full-time for a day or two, or a few mornings in a row, or on-and-off over a week or two.
Step 2: Make a sound association
Respond by making a simple sound for the duration of their pee or poo, so they learn to associate this with the muscles they're using at that time. Then, once finished, acknowledge what happened for them in a relaxed and friendly tone, e.g. "you’ve peed, that must feel good". It is part of the process for their learning and they understand language and especially your tone much earlier than you think.
A popular sound for pee is 'pssss', and for poo it can be another like 'uh uh'.
Step 3: Think about what you've learned
After a quick gentle clean, consider what you've learned. Were there any specific behaviours beforehand? How long since the previous pee or poo? How does that relate to when they last ate or drank? Note down the time if that helps. Most toddlers pee frequently in the morning (e.g. every 1-2 hours) and less later in the day.
You can acknowledge and reflect anytime you are sure a pee or poo is happening (even with nappy on)
Babycarrying
Most toddlers still love to be held close and comfortable, so baby-carrying using a stretchy wrap is a great way to support them. When you are carrying your toddler very close to your body you get very attuned to their movements and can quickly recognise an 'I need a toilet' squirm to get out of the carrier. When they were babies they were born with an instinct not to want to pee on you (to avoid soiling their 'nest') but by this age that may have been lost through lots of nappy use. It's good practice to offer when you take them out of the carrier (if they've been in there for a while) just to see. If you experience a phantom pee (hot sensation as if you're being peed on, but aren't) it's worth offering the potty to see if they need to go. There are many types of carriers and you may prefer different ones for different activities. Personally, I preferred a Tula Explore for longer walks; a moby wrap for cosy time at home; and a woven ring sling for out of the car and popping into a shop and back again.