(please excuse me for not yet making a picture for the activity below)
You can buy a protractor at a store and hang a washer or paper clip from a string tied to the protractor and use the equation at the bottom of the page

Materials: Paper Clip and cardboard

Step 1: Print and cut-out along dotted lines.

Step 2: Glue to cardboard of same size.

Step 3: Punch hole where indicated.

Step 4: Bend paper clip as shown, and put through hole.

1. Observer, not launcher, walks away from launch pad at least a couple of hundred feet. To be as precise as possible, unroll a pre-measured roll string. The ideal distance to walk is about equal to the height expected (i.e. the angle measured will be near 45 degrees, if expected height is achieved).

2. After rocket launch, Observer looks down sight line (indicated on drawing), and when rocket reaches it's peak altitude, Observer notes the angle made by the hanging paper clip.

3. Calculate height = sin(angle) * distance_from_pad