Reading MCU and component datasheets is a whole new skill to me.

I have a wire that I want to monitor as high/low (1/0) voltage, using a PIC10F200. (datasheet)

I presume somewhere in the datasheet the min/max voltage an IO pin configured as input guarantees to read 1 or 0 is defined.

Can someone teach me how to find these values in the datasheet for this particular PIC?

Most microcontroller datasheets have a section "(DC) electrical characteristics", usually after the functional description of the device. Electrical characteristics usually start with "Absolute Maximum Ratings" (AMR), which you may want to look at too, and especially the note below the table.

You'll see that there are different tables for "Industrial" and "Extended" parts. This refers to their operation temperature range; Industrial is up to 85 °C, extended up to 125 °C. You'll probably want Industrial, which is the standard range. Then on page 67 (Acrobat Reader page 69) you'll find $V_{IL}$ and $V_{IH}$, which is the maximum input voltage which will be read as a low level and the minimum which will be read as a high level. Most microcontrollers only have one type of input, and then there's just one value for each, usually as a fraction of the supply voltage.

For the standard type I/Os you have

$V_{IL}$ is 0.15 $V_{DD}$ maximum, so for a 5 V supply that's 0.75 V.
$V_{IH}$ is 0.25 $V_{DD}$ + 0.8 V minimum, for a 5 V supply voltage that's 2.05 V.

Notes:
1. At the top of the table the conditions for these parameters are mentioned. Sometimes that includes power supply voltage range, but here it's only temperature range.
2. The equation for $V_{IH}$ with the constant factor of 0.8 V is a bit unusual. You'll more often find a value like 0.7 $V_{DD}$ = 3.5 V.

Page 67 of the provided datasheet.

Parameters Vih and Vil specify the voltages you asked for.

To be more specific :

• Vih specifies the voltage range that guaranties that the IO reads a "1"
• Vil specifies the voltage range that guaranties that the IO reads a "0"

Starting on page 65 are tables containing the DC characteristics. There is usually always a "absolute max" rating for each characteristic listed. It is generally a good idea to have the input pulled up to the supply voltage so that when it is high it is at Vdd and when it is low it is pulled to ground. That ensures there are no floating inputs which could lead to incorrect results.