diff --git a/cibuildwheel/linux.py b/cibuildwheel/linux.py
index e3cd9fa5..b2569692 100644
--- a/cibuildwheel/linux.py
+++ b/cibuildwheel/linux.py
@@ -70,9 +70,14 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
                 {environment_exports}
 
                 # check the active python and pip are in PYBIN
-                # if `test` returns false, the script will exit due to errexit
-                test "$(which pip)" = "$PYBIN/pip"
-                test "$(which python)" = "$PYBIN/python"
+                if [ "$(which pip)" != "$PYBIN/pip" ]; then
+                    echo "cibuildwheel: python available on PATH doesn't match our installed instance. If you have modified PATH, ensure that you don't overwrite cibuildwheel's entry or insert python above it."
+                    exit 1
+                fi
+                if [ "$(which python)" != "$PYBIN/python" ]; then
+                    echo "cibuildwheel: pip available on PATH doesn't match our installed instance. If you have modified PATH, ensure that you don't overwrite cibuildwheel's entry or insert pip above it."
+                    exit 1
+                fi
 
                 if [ ! -z {before_build} ]; then
                     sh -c {before_build}
diff --git a/cibuildwheel/macos.py b/cibuildwheel/macos.py
index b0bbc18d..0e95ff28 100644
--- a/cibuildwheel/macos.py
+++ b/cibuildwheel/macos.py
@@ -2,6 +2,7 @@ import os
 import shlex
 import shutil
 import subprocess
+import sys
 import tempfile
 from collections import namedtuple
 from glob import glob
@@ -137,15 +138,20 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
         # check what version we're on
         call(['which', 'python'], env=env)
         call(['python', '--version'], env=env)
-        # TODO Cleanup/merge with above `call` once we have `subprocess.run` after dropping Python 2 support?
-        assert subprocess.check_output(['which', 'python'], env=env, universal_newlines=True).strip() == '/tmp/cibw_bin/python'
+        which_python = subprocess.check_output(['which', 'python'], env=env, universal_newlines=True).strip()
+        if which_python != '/tmp/cibw_bin/python':
+            print("cibuildwheel: python available on PATH doesn't match our installed instance. If you have modified PATH, ensure that you don't overwrite cibuildwheel's entry or insert python above it.", file=sys.stderr)
+            exit(1)
 
         # install pip & wheel
         call(['python', get_pip_script], env=env, cwd="/tmp")
         assert os.path.exists(os.path.join(installation_bin_path, 'pip'))
+        call(['which', 'pip'], env=env)
         call(['pip', '--version'], env=env)
-        # TODO Cleanup/merge with above `call` once we have `subprocess.run` after dropping Python 2 support?
-        assert subprocess.check_output(['which', 'pip'], env=env, universal_newlines=True).strip() == '/tmp/cibw_bin/pip'
+        which_pip = subprocess.check_output(['which', 'pip'], env=env, universal_newlines=True).strip()
+        if which_pip != '/tmp/cibw_bin/pip':
+            print("cibuildwheel: pip available on PATH doesn't match our installed instance. If you have modified PATH, ensure that you don't overwrite cibuildwheel's entry or insert pip above it.", file=sys.stderr)
+            exit(1)
         call(['pip', 'install', '--upgrade', 'setuptools', 'wheel', 'delocate'], env=env)
 
         # setup target platform, only required for python 3.5
diff --git a/cibuildwheel/windows.py b/cibuildwheel/windows.py
index 8a8aff97..bd93e1db 100644
--- a/cibuildwheel/windows.py
+++ b/cibuildwheel/windows.py
@@ -1,6 +1,7 @@
 import os
 import shutil
 import subprocess
+import sys
 import tempfile
 from collections import namedtuple
 from glob import glob
@@ -141,15 +142,19 @@ def build(project_dir, output_dir, test_command, test_requires, test_extras, bef
         simple_shell(['where', 'python'], env=env)
         simple_shell(['python', '--version'], env=env)
         simple_shell(['python', '-c', '"import struct; print(struct.calcsize(\'P\') * 8)"'], env=env)
-        # TODO Cleanup/merge with above `simple_shell` once we have `subprocess.run` after dropping Python 2 support?
-        assert subprocess.check_output(['where', 'python'], env=env, universal_newlines=True).splitlines()[0] == os.path.join(installation_path, 'python.exe')
+        where_python = subprocess.check_output(['where', 'python'], env=env, universal_newlines=True).splitlines()[0].strip()
+        if where_python != os.path.join(installation_path, 'python.exe'):
+            print("cibuildwheel: python available on PATH doesn't match our installed instance. If you have modified PATH, ensure that you don't overwrite cibuildwheel's entry or insert python above it.", file=sys.stderr)
+            exit(1)
 
         # make sure pip is installed
         if not os.path.exists(os.path.join(installation_path, 'Scripts', 'pip.exe')):
             simple_shell(['python', get_pip_script], env=env, cwd="C:\\cibw")
         assert os.path.exists(os.path.join(installation_path, 'Scripts', 'pip.exe'))
-        # TODO Cleanup/merge with above `simple_shell` once we have `subprocess.run` after dropping Python 2 support?
-        assert subprocess.check_output(['where', 'pip'], env=env, universal_newlines=True).splitlines()[0] == os.path.join(installation_path, 'Scripts', 'pip.exe')
+        where_pip = subprocess.check_output(['where', 'pip'], env=env, universal_newlines=True).splitlines()[0].strip()
+        if where_pip.strip() != os.path.join(installation_path, 'Scripts', 'pip.exe'):
+            print("cibuildwheel: pip available on PATH doesn't match our installed instance. If you have modified PATH, ensure that you don't overwrite cibuildwheel's entry or insert pip above it.", file=sys.stderr)
+            exit(1)
 
         # prepare the Python environment
         simple_shell(['python', '-m', 'pip', 'install', '--upgrade', 'pip'], env=env)