diff --git a/ThickNumbersInterfaceTests/src/ThickNumbersInterfaceTests.jl b/ThickNumbersInterfaceTests/src/ThickNumbersInterfaceTests.jl
index dd04f7f..84bd08b 100644
--- a/ThickNumbersInterfaceTests/src/ThickNumbersInterfaceTests.jl
+++ b/ThickNumbersInterfaceTests/src/ThickNumbersInterfaceTests.jl
@@ -28,6 +28,7 @@ function test_required(f::Function, @nospecialize(TN), @nospecialize(Ts=nothing)
     lo, hi = 1/3, nextfloat(2/3)
     x = f(TN, lo, hi)
     @test typeof(x) <: TN
+    @test isthick(TN)
     @test lo ∈ x
     @test hi ∈ x
     @test lo ≈ loval(x)
diff --git a/docs/src/developers.md b/docs/src/developers.md
index 427f410..c6ac0a0 100644
--- a/docs/src/developers.md
+++ b/docs/src/developers.md
@@ -50,3 +50,21 @@ be sure to use `[compat]` bounds to specify the major version number of `ThickNu
 ## Features provided by subtyping ThickNumber
 
 See the [User API](@ref).
+
+## Implementing the interface without subtyping
+
+Subtype `ThickNumber` to inherit the generic methods in the [User API](@ref).
+
+A type with another supertype can implement the interface by defining [`isthick`](@ref):
+
+```julia
+ThickNumbers.isthick(::Type{<:MyType}) = true
+```
+
+It must implement [`loval`](@ref), [`hival`](@ref), [`lohi`](@ref), [`basetype`](@ref),
+supported arithmetic, and any needed derived methods. These may include [`mid`](@ref),
+[`wid`](@ref), [`rad`](@ref), [`mag`](@ref), [`mig`](@ref), [`hull`](@ref),
+[`emptyset`](@ref), [`isempty_tn`](@ref), [`isnan_tn`](@ref), [`isinf_tn`](@ref),
+[`isfinite_tn`](@ref), and `iszero`.
+
+Use `isthick(x)` to recognize both kinds of implementation.
diff --git a/docs/src/index.md b/docs/src/index.md
index 59af697..27e3f70 100644
--- a/docs/src/index.md
+++ b/docs/src/index.md
@@ -64,6 +64,7 @@ You can also check a few basic properties, like whether the values contained in
 ### Type information
 
 - [`valuetype(X)`](@ref): return the type of numbers contained in `X` (e.g., `Float64`)
+- [`isthick(X)`](@ref): return `true` if `X` (or its type) implements the ThickNumbers interface
 
 ### Generic constructors
 
@@ -92,6 +93,7 @@ For an explanation of why these aren't just `==`, `<`, etc, read [The Fundamenta
 - [`issupset_tn`](@ref) replaces `⊇` (unicode: `⫄`)
 - [`is_strict_supset_tn`](@ref) replaces `⊃` (unicode: `⪾`)
 - [`hull`](@ref) creates a number that contains its arguments
+- [`isempty_tn(X)`](@ref) replaces `isempty(X)`
 
 ### API reference
 
diff --git a/docs/src/user_api.md b/docs/src/user_api.md
index 870dcaa..9c44673 100644
--- a/docs/src/user_api.md
+++ b/docs/src/user_api.md
@@ -9,6 +9,7 @@ violate the interface requirements.
 
 ```@docs
 valuetype
+isthick
 ```
 
 ## Query functions
@@ -54,6 +55,7 @@ See also [IntervalSets](https://github.com/JuliaMath/IntervalSets.jl) for a more
 in(::Real, ::ThickNumber)
 hull
 Base.isempty(::ThickNumber)
+isempty_tn
 issubset_tn
 issupset_tn
 is_strict_subset_tn
diff --git a/src/ThickNumbers.jl b/src/ThickNumbers.jl
index 4bf100a..ece4093 100644
--- a/src/ThickNumbers.jl
+++ b/src/ThickNumbers.jl
@@ -5,7 +5,7 @@ using LinearAlgebra
 export ThickNumber, FPTNException
 
 # Traits
-export valuetype, basetype
+export valuetype, basetype, isthick
 
 # These mimic IEEE Std 1788-2015, Table 9.2, but with `inf` and `sup`
 # replaced by names that do not imply true bounds.
@@ -16,7 +16,7 @@ export valuetype, basetype
 export lohi, midrad, loval, hival, mid, wid, rad, mag, mig
 
 # Set operations
-export emptyset, hull, issubset_tn, ⫃, is_strict_subset_tn, ⪽, issupset_tn, ⫄, is_strict_supset_tn, ⪾
+export emptyset, hull, issubset_tn, ⫃, is_strict_subset_tn, ⪽, issupset_tn, ⫄, is_strict_supset_tn, ⪾, isempty_tn
 
 # Operators
 export isequal_tn, iseq_tn, ≐, isapprox_tn, ⩪, isless_tn, ≺, ≻, ⪯, ⪰
@@ -59,6 +59,26 @@ Float64
 valuetype(::Type{TN}) where TN<:ThickNumber{T} where T = T
 valuetype(x::ThickNumber) = valuetype(typeof(x))
 
+"""
+    isthick(::Type{T}) where T
+    isthick(x)
+
+Return whether a type implements the ThickNumbers interface.
+
+# Default implementation
+
+Subtypes of `ThickNumber` return `true` by default:
+
+    isthick(::Type{T}) where T = T <: ThickNumber
+    isthick(x) = isthick(typeof(x))
+
+Types with another supertype can implement the interface by defining
+`isthick(::Type{<:SomeType}) = true`. See [Implementing the interface without
+subtyping](@ref).
+"""
+isthick(::Type{T}) where T = T <: ThickNumber
+isthick(x) = isthick(typeof(x))
+
 # Functions that must be defined by subtypes
 
 """
@@ -403,12 +423,19 @@ The converse of [`is_strict_subset_tn`](@ref).
 is_strict_supset_tn(a::ThickNumber, b::ThickNumber) = is_strict_subset_tn(b, a)
 const ⪾ = is_strict_supset_tn
 
+"""
+    isempty_tn(x::ThickNumber)
+
+Return whether the span of `x` is empty (`hival(x) < loval(x)`).
+"""
+isempty_tn(x::ThickNumber) = hival(x) < loval(x)
+
 """
     isempty(x::ThickNumber)
 
 Returns `true` if the span of `x` is empty (`hival(x) < loval(x)`), `false` otherwise.
 """
-Base.isempty(x::ThickNumber) = hival(x) < loval(x)
+Base.isempty(x::ThickNumber) = isempty_tn(x)
 
 function Base.isdisjoint(a::ThickNumber, b::ThickNumber)
     (isempty(a) || isempty(b)) && return true
diff --git a/test/runtests.jl b/test/runtests.jl
index 6d035f7..36b0e2d 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -10,6 +10,10 @@ using MidRadArith
     # Test all the operations defined in ThickNumbers
     x, y = Interval(1.0, 3.0), Interval(-1.0, 3.0)
     @test valuetype(x) === valuetype(typeof(x)) === Float64
+    @test !isthick(Float64)
+    @test !isthick(3.0)
+    @test isthick(Interval{Float64})
+    @test isthick(x)
     @test loval(x) === 1.0
     @test hival(x) === 3.0
     @test mid(x) === 2.0
@@ -105,6 +109,10 @@ using MidRadArith
     @test emptyset(Interval{Float32}) === Interval(Inf32, -Inf32)
     @test emptyset(Interval{Float64}) === Interval(Inf, -Inf)
     @test emptyset(Interval(1.0, 2.0)) === Interval(Inf, -Inf)
+    @test isempty_tn(emptyset(Interval{Float64}))
+    @test !isempty_tn(x)
+    @test isempty(x) == isempty_tn(x)
+    @test isempty(emptyset(Interval{Float64})) == isempty_tn(emptyset(Interval{Float64}))
     @test isfinite_tn(Interval(1, 2))
     @test_throws FPTNException isfinite(Interval(1, 2))
     @test !isfinite_tn(Interval(1, Inf))